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Thread: Low output from Eden WT800 in Bridge Mode

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    Low output from Eden WT800 in Bridge Mode

    Hello all.
    Okay, I've been lucky with trial, and error troubleshooting, and eventually repairing circuits. Don't want to waste anyone's time here with stuff I should already know. But I just can't figure this one out.
    Getting weirdness from bridge mode.
    First thing I did was replace the open 27K feedback resistor that had failed open.
    In bridge mode, I'm getting a larger signal on negative side (inverted right channel) than I'm getting from the positive side (non-inverted left channel).
    Both right, and left channels are putting out the correct wattage in stereo mode.
    Bridge switch seems fine. The R channel input is being muted by JFETS when in bridge mode. Getting signal from L channel output at 12K/27K junction.
    When scoping output negative terminal I'm getting 36V RMS before clipping
    Positive terminal is reading 22V rms.
    At this point, I can apply more signal and get 36V on Pos terminal as well, but the negative side is clipping.
    With the above 36/22V RMS scenario, I'm reading 58V across NEG and Positive with DVM.
    Obviously, far below the specified 800 watts for bridged into 8 ohm load.
    Using electric stove hotplates parallel to get a perfect 8 ohms.
    Hoping someone can see something obvious here.
    I appreciate, and humbly await any response to guide me.
    Thanks,
    Stan
    Attached Thumbnails Attached Thumbnails eden_wt500_wt600_wt800_sch.pdf  

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    Do you have your scope ground disconnected when you measure with your DVM from neg to pos ?

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    No grounds connected on both probes.
    Thanks for asking. Would upping the 27K resistor keep right channel from going into clipping? The circuit actually had a 27.4K on board.I'm thinking not?
    I really appreciate you taking the time to write back.
    Cheers!

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    Not balanced

    Quote Originally Posted by Stan Keith View Post
    No grounds connected on both probes.
    Thanks for asking. Would upping the 27K resistor keep right channel from going into clipping? The circuit actually had a 27.4K on board.I'm thinking not?
    I really appreciate you taking the time to write back.
    Cheers!
    Schematic of more recent recent model attached, but same principle with the 12K/27K.
    Don't quite understand how this works, but could I balance neg/positive by tweaking that section?
    Thanks
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    I'm locked out of my shop at this time (in California), though I do have my database here. I forget what the power amp supply voltages are.....above +/-70V, less than +/- 80V if memory serves. The PCB layout on the power amp module is the same for the left Ch (right side viewed from the front) and right Ch (left side viewed from the front). You've already verified you're getting the same output level in Stereo Mode. The Bridge Mono mode is created by taking the output from the left Ch output, passing it thru the attenuator network which uses a 12k not on the PCB, and a 27k R16 on the Right Ch PCB, total 39k, which is the same value as the feedback resistor from the output stage to the Inverting input of the power amp diff pair. That makes the Right CH pwr amp an Inverting unity gain amp.

    Check what you get under No Load condition. Since you don't have a differential input Audio Analzyer (I presume), you still have your DMM in AC Mode. Set an arbitrary output level, of, say 10VAC on both channels in Stereo Mode. Switch to Bridge Mode, and check that you still have that same output level on both the Left and Right Outputs (relative to Ground). Should be the same. Connect the DMM between the Bridge Output binding posts (or Neutrik NL4, if you don't have the BP's). Should be 20V RMS. And, with scope, ground lead attached to spkr gnd, you should have the same level on both chennels, with the Right Ch opposite phase from the Left Ch in this mode. Increase the output until you see clipping in Stereo Mode again, still no load. Note your AC Output voltage. Switch to Bridge Mono, and see if your two channels are still the same, and if across the L-R outputs, you have twice the output voltage. If not, you can set up the Left Ch heat sink module so it's the same as the Right Ch, adding the Bridge Mode connection parts. I forget if that Bridge connection plugs into the amp board or into the power supply board, with the bridge input wire attached to the PCB. I didn't record close-up images of the Eden WT800 modules/chassis/wiring to give me those details.

    If you're NOT getting the same output voltage, UNLOADED, then yes, we're dealing with the external 12k resistor from the Left Ch output connector and the 27k resistor on the amp PCB for the Right Ch. If you are getting identical output levels at clip in both Stereo and Bridge Mono mode, and, twice that between the Left/Right outputs in Bridge, turn the output master down, and connector your load across Left/Right Output (only in Bridge Mono mode).

    If you're still getting the imbalance, that's when I'd swap the two amp modules in the chassis, taking care to connect the former Left Ch correctly for Bridge Mono with the chassis wiring. See if the problem moves under load. I'm assuming the problem occurs under load and NOT unloaded.

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    Last edited by nevetslab; 03-26-2020 at 06:00 AM.
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    Thanks for the detailed information, nevetslab.I really appreciate your time it took to post.
    It appears that the 27K I soldered in parallel with what I thought was an open resistor, was indeed good, and I was actually dealing with a resistor closer to 14 ohms at that point. OOPS
    I measured several times before making the call. Thought I had DVM probes on clean leads, but I think all the damn epoxy eden put on resistor still had some on where I was measuring.
    The whole time I thought there was something more to this simple way of inverting signal, but the schematic is so unreadable, I kept going down different rabbit holes.
    That said, how important is it that both +and- outputs are exactly the same?
    I was able to get them close to exact using a trimpot for the 27K. I'm getting about 36V RMS before clip from both +and- jacks. Using 2 ungrounded probes with a Tektronix scope This would equate to 648 watts into an 8 ohm load. Cheapish true RMS meter measures less across +and- jacks. So I'm thinking the scope should be trusted more?
    Is this an acceptable ballpark wattage output, or should I investigate further?
    Thanks again,
    Stan

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    Last edited by Stan Keith; 03-26-2020 at 01:40 AM. Reason: spelling grammer

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    At this point I'm thinking your stove elements may not be acting the same with signal as without.
    Have you tried swapping them around to see if the imbalance moves to the other side?
    To verify their resistance under load you could measure current and voltage at the same time and calculate.

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    Quote Originally Posted by Stan Keith View Post
    Thanks for the detailed information, nevetslab.I really appreciate your time it took to post.
    It appears that the 27K I soldered in parallel with what I thought was an open resistor, was indeed good, and I was actually dealing with a resistor closer to 14 ohms at that point. OOPS
    I measured several times before making the call. Thought I had DVM probes on clean leads, but I think all the damn epoxy eden put on resistor still had some on where I was measuring.
    The whole time I thought there was something more to this simple way of inverting signal, but the schematic is so unreadable, I kept going down different rabbit holes.
    That said, how important is it that both +and- outputs are exactly the same?
    I was able to get them close to exact using a trimpot for the 27K. I'm getting about 36V RMS before clip from both +and- jacks. Using 2 ungrounded probes with a Tektronix scope This would equate to 648 watts into an 8 ohm load. Cheapish true RMS meter measures less across +and- jacks. So I'm thinking the scope should be trusted more?
    Is this an acceptable ballpark wattage output, or should I investigate further?
    Thanks again,
    Stan
    Good find on the pair of 27k resistors. I've been down that road before. How important is it for the two power amps being equal? Mostly has to do with current each amp is contributing to the load. Considering we're really using this for amplifying music waveforms, which has NOTHING to do with symmetrical sine waveforms, the current being passed thru the load is always changing. It has more to do with headroom than anything else, I think. I'm sure more exacting answers to that will be forthcoming it this is way off base.

    On your Tektronix scope, you can place the two channels into single ch differential mode (each scope channel's sensitivity needs to be the same, one set to invert, as all my Tek scopes have, along with most or all other two or more channel scopes). So, you CAN measure differentially across the Eden's bridge mono output, and use GND on one of the probes to the Eden's output Gnd. And, yes, it will have far greater bandwidth than your DMM's True RMS AC Mode. Enough so that you can sweep out with your oscillator/signal generator to at least 100kHz, though I wouldn't be doing that at full power. 1 to 10W level more than adequate to see the amp's bandwidth. Measuring accurately the Average AC voltage with the scope is not as accurate as with the DMM, since you're taking the p-p magnatude, and dividing that by square root of 2 (1.414). I'm guessing your DMM's signal bandwidth isn't real high. Easy enough to check though.

    So, 36V per amp channel, or 72V in Bridge.....that into an 8 ohm load is ~648W, while 80V out in Bridge is 800W. So, you're in the ballpark. If you're not using a Variac and AC Line Monitor to keep the AC mains at 120VAC at the power amp source, that could be why its a little low. So, it sounds like you're working ok.

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    Last edited by nevetslab; 03-27-2020 at 12:45 AM.
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    Quote Originally Posted by g1 View Post
    At this point I'm thinking your stove elements may not be acting the same with signal as without.
    Have you tried swapping them around to see if the imbalance moves to the other side?
    To verify their resistance under load you could measure current and voltage at the same time and calculate.
    That is something I need to investigate.Thanks for that!

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    Quote Originally Posted by nevetslab View Post
    Good find on the pair of 27k resistors. I've been down that road before. How important is it for the two power amps being equal? Mostly has to do with current each amp is contributing to the load. Considering we're really using this for amplifying music waveforms, which has NOTHING to do with symmetrical sine waveforms, the current being passed thru the load is always changing. It has more to do with headroom than anything else, I think. I'm sure more exacting answers to that will be forthcoming it this is way off base.

    On your Tektronix scope, you can place the two channels into single ch differential mode (each scope channel's sensitivity needs to be the same, one set to invert, as all my Tek scopes have, along with most or all other two or more channel scopes). So, you CAN measure differentially across the Eden's bridge mono output, and use GND on one of the probes to the Eden's output Gnd. And, yes, it will have far greater bandwidth than your DMM's True RMS AC Mode. Enough so that you can sweep out with your oscillator/signal generator to at least 100kHz, though I wouldn't be doing that at full power. 1 to 10W level more than adequate to see the amp's bandwidth. Measuring accurately the Average AC voltage with the scope is not as accurate as with the DMM, since you're taking the p-p magnatude, and dividing that by square root of 2 (1.414). I'm guessing your DMM's signal bandwidth isn't real high. Easy enough to check though.

    So, 36V per amp channel, or 72V in Bridge.....that into an 8 ohm load is ~720W, while 80V out in Bridge is 800W. So, you're in the ballpark. If you're not using a Variac and AC Line Monitor to keep the AC mains at 120VAC at the power amp source, that could be why its a little low. So, it sounds like you're working ok.
    Thanks. Put amp back together. But I'm going to put back on bench, and adjust variac to stay at 120 V as I observe scope. BTW, I'm using the RMS feature on my Tektronix 2430A. I believe it has a bandwidth of 100MHz. This is not as good as a DMM for measuring RMS? Until now, I was just injecting a 1KHz sinewave, and not doing a full band sweep. I will do a sweep at 40Hz and up. Thanks guys for giving lots of food for thought. I'm starving! ha

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    Quote Originally Posted by Stan Keith View Post
    Thanks. Put amp back together. But I'm going to put back on bench, and adjust variac to stay at 120 V as I observe scope. BTW, I'm using the RMS feature on my Tektronix 2430A. I believe it has a bandwidth of 100MHz. This is not as good as a DMM for measuring RMS? Until now, I was just injecting a 1KHz sinewave, and not doing a full band sweep. I will do a sweep at 40Hz and up. Thanks guys for giving lots of food for thought. I'm starving! ha
    I've never read the specs on their 2430A scope. The scope's vertical amplifiers have a BW of 100MHz, though I doubt if the RMS DMM in the system does. Should be in your Specs in your Owner's manual (wherever that is now). Having 100MHz BW on an RMS Multimeter.....that's in the expensive class of instruments. My Marconi 2610 has a 25MHz BW in the 200mV-2V range, drops down to 5MHz on 20V range, & 20kHz on the 200V range. My HP3400A I think is a 100MHz BW instrument, as was the Fluke 8920A that I once owned. Sold that. I'd guess your DMM of the 2430A is at least 20kHz.

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    As long as the signal is a pure sinewave there shouldn't be a difference between equivalent RMS and "True RMS". Extended bandwidth above the signal frequency is only required for the higher harmonics that don't exist in a sinewave.

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    So, hooked up to variac with current meter. This thing draws about 14 amps. The sine waveform is starting to clip at 32V MS on each leg as the mains (AC monitored with DMM) drops to 113 V.
    Unfortunately, I think I'm not getting good readings because the variac is rated for 5 amps ( I primarily do vacuum tube repairs). At least that's what the fuse is. Taking a chance when measuring just for a few seconds.
    Was able to ride AC with variac to keep it at 120V. About the same wattage as before- I did gain a few watts at 1KHz, not so much with 40Hz. I believe 72x72/8 = 648 watts, right? The 72 figure is the bridged RMS voltage of +and- neg summed.
    At 1KHz I'm getting more like 74x74/8 = 684 watts.
    Now, I have to think about my equipment not up to par measuring high current amps like this.
    I would think if it's quiet, and not distorting, the amp is probably okay at that point? Or could there be something more subtle that's sapping wattage?
    Thanks again in advance for any info on this.
    Stan

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    Quote Originally Posted by g1 View Post
    At this point I'm thinking your stove elements may not be acting the same with signal as without.
    Have you tried swapping them around to see if the imbalance moves to the other side?
    To verify their resistance under load you could measure current and voltage at the same time and calculate.
    Quote Originally Posted by g1 View Post
    At this point I'm thinking your stove elements may not be acting the same with signal as without.
    Have you tried swapping them around to see if the imbalance moves to the other side?
    To verify their resistance under load you could measure current and voltage at the same time and calculate.
    Please tell me if I have hooked this up incorrectly-
    Hooked up DMM for AC amps in series with + side of amp to + terminal on dummy load. Brought up variac while observing both +and- RMS V. (inverted from each other) Both were at 22V, and I was drawing 6A AC current thru + leg..
    However, when hooking up DMM exactly the same way on - side, the two signals are in phase. What the #&^?
    I'm missing something fundamental here that would like to be schooled on.
    Also, for the simple ohms law calculation, is the 6 amps I initially measured the total of both +and-outputs? So would it be 22RMS each side = 44V/6 amps= 7.3 ohms? Or would I double the current as well? 44/12 = 3.6 ohms?
    if so, that would almost explain a few things.
    But what of the two waveforms being in phase when hooking up DMM as described before?
    I am a sponge. Please dip me in your knowledge.

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    However, when hooking up DMM exactly the same way on - side, the two signals are in phase. What the #&^?
    It is not clear how you measured and what - side means. I suggest to post a drawing that shows how you connected the load and your meter.

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    Quote Originally Posted by Helmholtz View Post
    It is not clear how you measured and what - side means. I suggest to post a drawing that shows how you connected the load and your meter.
    Quote Originally Posted by Helmholtz View Post
    It is not clear how you measured and what - side means. I suggest to post a drawing that shows how you connected the load and your meter.
    - is the inverted side of bridged output. I have probes on pos and negative outputs. Trying to determine if my stove top heating elements are the same ohms under load per G1 (lemmy) suggestion.
    Attached diagram. As I understand it, I'm measuring RMS voltage across 8 ohm load, and dividing by current measured. (as seen in attached pic).
    Thanks for your replay,Helmholtz.
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    Attached diagram. As I understand it, I'm measuring RMS voltage across 8 ohm load, and dividing by current measured. (as seen in attached pic).
    Ok, clear so far and correct. I understand that + and - denominate the hot terminals of left and right outputs in bridged mode. There is only one voltage and one current. What caused the confusion?

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    Quote Originally Posted by Helmholtz View Post
    Ok, clear so far and correct. I understand that + and - denominate the hot terminals of left and right outputs in bridged mode. There is only one voltage and one current. What caused the confusion?
    I have a scope lead on either side of load to observe inverted, and in phase sinewaves. When I move the DMM to measure current on negative side of load, the signal that was inverted is now in phase. Why would it do that? The DMM can't invert signal, can it?
    Hope this is clear.

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    Quote Originally Posted by Stan Keith View Post
    I have a scope lead on either side of load to observe inverted, and in phase sinewaves. When I move the DMM to measure current on negative side of load, the signal that was inverted is now in phase. Why would it do that? The DMM can't invert signal, can it?
    Hope this is clear.
    Sorry not clear. Current on both sides of the load resistor must be the same. Please show how you connected the scope. I assume you use two scope probes connected directly to right and left outputs?

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    I think in this setup with the 5-Amp Variac, it's core is saturating, since your amp at full power is pulling too much current thru it. What did you measure unloaded in Bridge Mode?

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    Quote Originally Posted by Stan Keith View Post
    I have a scope lead on either side of load to observe inverted, and in phase sinewaves. When I move the DMM to measure current on negative side of load, the signal that was inverted is now in phase. Why would it do that? The DMM can't invert signal, can it?
    Hope this is clear.
    I just downloaded the Tek 2430A manual, and what I don't see is independent DMM connections. I'm lead to believe the DMM function is being made via the scope probe leads. I didn't read thru the manual closely....does this have a Ch 2 & C4, which also serve as the DMM inputs? If they're not floating, as an independed battery powered DMM would be, it may be shorting the Right Ch output. If you have an independent DMM to measure the AC current with, as shown in your diagram, then there shouldn't be phase shift.

    Tek 2430A Scope User's Manual.pdf

    Read thru the manual, and....unless I'm mistaken, it does NOT have an independent floating DMM like some of their earlier scopes had (Tek 465, 475 had such an option decades ago). And, only two vertical channels, so you're getting the math processing power for scope measurements.

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    Quote Originally Posted by nevetslab View Post
    I think in this setup with the 5-Amp Variac, it's core is saturating, since your amp at full power is pulling too much current thru it. What did you measure unloaded in Bridge Mode?
    I think it's impossible to saturate a transformer core by AC load current.

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    Quote Originally Posted by Helmholtz View Post
    Sorry not clear. Current on both sides of the load resistor must be the same. Please show how you connected the scope. I assume you use two scope probes connected directly to right and left outputs?
    switching DMM to negative side makes otherwise inverted number 2 sinewave in phase with sinewave 1.(shown in attached picture)
    Scope leads are consistent. probe 1 on pos, and probe 2 on neg.
    Just curious why it would do that it's still just in series with everything. Should be the same waveform, right?
    Vielen Dank!
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    Quote Originally Posted by Helmholtz View Post
    I think it's impossible to saturate a transformer core by AC load current.
    Well, if Stan's output voltage has fallen to 113VAC at full power, when I had 120V at low current, we're causing voltage sag. Maybe Saturation is the wrong term here? (sorry about that)

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    Quote Originally Posted by nevetslab View Post
    Well, if Stan's output voltage has fallen to 113VAC at full power, when I had 120V at low current, we're causing voltage sag. Maybe Saturation is the wrong term here? (sorry about that)
    That would be voltage drop across the winding's DCR.

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    Quote Originally Posted by Helmholtz View Post
    That would be voltage drop across the winding's DCR.
    Ah....of course. My mistake.

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    Quote Originally Posted by Stan Keith View Post
    switching DMM to negative side makes otherwise inverted number 2 sinewave in phase with sinewave 1.(shown in attached picture)
    Scope leads are consistent. probe 1 on pos, and probe 2 on neg.
    Just curious why it would do that it's still just in series with everything. Should be the same waveform, right?
    Vielen Dank!
    I don't see a reason why the phase of one scope channel should jump if you leave both probes connected to left and right outputs. Maybe some trigger instability. If the two outputs would actually be in phase, current would be zero.

    BTW, there is no reason to not connect the probe grounds to the amp's ground. Just never connect probe grounds to one of the (hot) output terminals.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by Helmholtz View Post
    I don't see a reason why the phase of one scope channel should jump if you leave both probes connected to left and right outputs. Maybe some trigger instability. If the two outputs would actually be in phase, current would be zero.

    BTW, there is no reason to not connect the probe grounds to the amp's ground. Just never connect probe grounds to one of the (hot) output terminals.
    Force of habbit with me. Ground-referrences the scope to the amp. So, I too am confused with Stan's connections. With an independent floating DMM (battery pwr'd meter), this would work.

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    Quote Originally Posted by Helmholtz View Post
    ......If the two outputs would actually be in phase, current would be zero.....
    ^^^^^^That! It almost has to be a triggering/measurement error of some kind. If phase was actually flipped on one side making the two outputs in phase, there would be no voltage drop across the load. I don't think that is the case, is it?

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    Quote Originally Posted by nevetslab View Post
    Force of habbit with me. Ground-referrences the scope to the amp. So, I too am confused with Stan's connections. With an independent floating DMM (battery pwr'd meter), this would work.
    Yes, the DVM is a battery powered unit I'm using to measure current.
    I obviously need to read manual, and gain a better understanding of the tektronix's operation.
    I've always used the RMS display on scope to measure amps output into load. Seems fairly accurate when cross checked with a good DMM.

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    Quote Originally Posted by Helmholtz View Post
    I don't see a reason why the phase of one scope channel should jump if you leave both probes connected to left and right outputs. Maybe some trigger instability. If the two outputs would actually be in phase, current would be zero.

    BTW, there is no reason to not connect the probe grounds to the amp's ground. Just never connect probe grounds to one of the (hot) output terminals.
    Hey, I thought I heard a click coming from somewhere! haha. I think it may have been from the scope. I was still reading current through meter, so I guess the phase didn't flip after all. Geez.
    I always believed that in bridge mode, one should NOT ground the scope probes.
    I'm learning some stuff here. Thanks!

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    Quote Originally Posted by nevetslab View Post
    I think in this setup with the 5-Amp Variac, it's core is saturating, since your amp at full power is pulling too much current thru it. What did you measure unloaded in Bridge Mode?
    Measured around ~95V across terminals (no load). I always thought I needed a load to determine anything. Can you explain what this no load voltage means?
    Man, you guys are inspiring me to bring out all my electronic books I've acquired but never read in depth.
    I'm in awe of the knowledge here on this forum.
    The reason it was brought to me is the guy thought it lacked in power.
    Just wondering if these measurements I've been posting are that of a correctly working bridged 800 watt amplifier.
    Sorry, I just don't work on a lot of there high current SS amps to be totally confident to tell owner to pick up.

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    Quote Originally Posted by Stan Keith View Post
    Measured around ~95V across terminals (no load). I always thought I needed a load to determine anything. Can you explain what this no load voltage means?
    Man, you guys are inspiring me to bring out all my electronic books I've acquired but never read in depth.
    I'm in awe of the knowledge here on this forum.
    The reason it was brought to me is the guy thought it lacked in power.
    Just wondering if these measurements I've been posting are that of a correctly working bridged 800 watt amplifier.
    Sorry, I just don't work on a lot of there high current SS amps to be totally confident to tell owner to pick up.
    One thing the no load measurement tells me is the amp IS working correctly in Bridge Mono mode. With no load, you don't have the copper losses that loose power under speaker-driven loads. You no doubt saw clean & significantly higher output swing vs the loaded condition. Without using your 5Amp Variac, you'd get higher output since there isn't the copper loss in the variac to loose AC mains voltage. As for the Eden WT800, I'm always amazed they work as well as they do, with so few output xstrs in the output stage. But, there is a VERY large Toroidal Power Xfmr in it, and they do kick butt quite well. And, like most solid state power amps, they do sound better in Bridge Mode...more head room! It might be the guy who brought it to you felt it lacked power relative to an even higher powered amp.

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    Quote Originally Posted by nevetslab View Post
    One thing the no load measurement tells me is the amp IS working correctly in Bridge Mono mode. With no load, you don't have the copper losses that loose power under speaker-driven loads. You no doubt saw clean & significantly higher output swing vs the loaded condition. Without using your 5Amp Variac, you'd get higher output since there isn't the copper loss in the variac to loose AC mains voltage. As for the Eden WT800, I'm always amazed they work as well as they do, with so few output xstrs in the output stage. But, there is a VERY large Toroidal Power Xfmr in it, and they do kick butt quite well. And, like most solid state power amps, they do sound better in Bridge Mode...more head room! It might be the guy who brought it to you felt it lacked power relative to an even higher powered amp.
    Speaking of copper loss, the mains voltage drops 5 volts as I near clipping. Difficult to get a good reading, so I guess I can tell him it's in the ballpark at this point.
    And maybe I should talk to my landlord about the 1930s area wiring. haha
    Thank you so much for taking time to guide me through this.
    I learned a few things.
    You, and the others who posted here are some cool dudes!

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    might be the guy who brought it to you felt it lacked power relative to an even higher powered amp.
    Actually, he thought his lower powered amps sounded louder. So that's why I'm reluctant to give back to him.
    Is it possible some of the current load is coming from "copper loss" within the toroidal transformer?
    Or, being that I'm getting a clean signal up to the shortcomings of my AC source, and measurement equipment, I'm likely in the "ballpark," and ready to hand back to owner?

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