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Thread: Opamp for fx loop

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    Opamp for fx loop

    I am looking at opamps that run on a heater circuit rectified. Here is what I was looking at:
    NJR 4558 Operational Amplifiers - Op Amps | Mouser
    Please let me know if these are good candidates and will running lower voltages affect sound quality. A friend loves the fx loop from the PV delta blues and I was gonna see how it sounds in a sloclone.

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    Supporting Member The Dude's Avatar
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    That should work fine. The 5532 would be lower noise. There are also companies that have kits like THIS ONE and others.

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    The 5532 would be lower noise.
    The low noise characteristics of the 5532 will only make a difference if the opamp is used in low impedance circuitry. Very quiet circuits can be built with this opamp, but only if you know what you're doing.

    Want a truly easy to use, all-round audio opamp? Use the 072. It's more than quiet enough for most purposes, and will perform well even if the circuit it is used in is not designed with utmost care.

    will running lower voltages affect sound quality
    The supply voltage determines the maximum signal level the circuit can handle. The higher your supply voltage, the higher your maximum signal level. Otherwise, it doesn't make any difference as long as you don't drop below the minimum rated supply voltage for the particular type of opamp.

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    An opamp spec sheet will specify an input common mode range. If you drive the input too hard, the output won't just clip, it will unpredictably slam the opposite rail. This sounds really bad. The input must be clamped so as not to allow the input to exceed the input common mode range. A low supply voltage means the input must be clamped at a lower voltage.

    There was recently a thread with a schematic for an effects loop, I think it was a Marshall 1987X. It ran off of a 30V supply derived from B+ and had input clamping diodes.

    Found it! Here's a link to the schematic, the effects loop is on page 4.
    http://www.webphix.com/schematic%20h..._50w_1987X.pdf

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    Last edited by loudthud; 10-22-2013 at 02:57 AM.
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    In theory, exceeding the opamps input common mode range might get you into trouble. In practice, this very rarely happens in real-life audio. Input common mode range is only ever an issue in non-inverting circuits, and these always have a gain of at least unity. If you're driving the stage so hard as to risk exceeding input common mode range, the output will probably be well into clipping already.

    That said, it's never a bad idea to apply some form of protection to inputs. Never assume no one would be so stupid as to connect a speaker jack to your line level input...

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    Lifetime Member Enzo's Avatar
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    If your friend loves the Delta Blues FX loop, it runs on -30v single sided, which would be the same as running on +15 and -15. So trying to run the same chip in a 6v circuit is not going to yield the same results. But that circuit doesn;t even use an op amp, it has a single emitter following transistor. So I don;t know how you can possibly compare them.

    4558 op amps are as common as it gets, they were a workhorse all through the 1980s. Newer versions like the 4580 serve the same these days.

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    You are correct enzo. The PV delta doesn't use the opamp for fx. I have to call him to see which amp it was because I know it was a opamp fx loop. I really hope that the loop will be transparent in the sloclone I built him. Which PV amps use opamp fx loops? Loudthud thanks for that link. I will have to pull from the B+ rail.

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    Old Timer J M Fahey's Avatar
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    What +/-V rails are we talking about?
    A normal center tapped filament winding (actual tap or 2 x 100 ohms resistors) has only 3.15 VAC per side, some +/-3V DC rails.
    MOST regular Op Amps (TL072/RC4558/NE5532) perform very poorly at such low supplies.
    Don't forget that output pins usually can't get closer than some 2 V to power rails.
    They have practically no breathing space here.

    One humble Op Amp I often use in such cases is LM358 .
    Not a stellar performer by any measure, but it was designed to work properly from a single supply +5V rail and +/*3V is fine.

    Although not the best Op Amp out there, it has saved me countless times and considering these are Lo-Fi , narrow band, distorted signals, it goes unnoticed.

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    If you have a center tapped heater secondary, you can use the circuit attached here to get a decent split supply for opamp circuitry. It uses a charge pump to obtain a raw DC supply of approx +/- 12V, with Zener diodes and series resistors to obtain a stable and sufficiently clean +/- 8V. It will easily supply 10mA, enough for most dual opamps. With lower values for the series resistors (100 or 120 ohms) 20mA should be possible.

    If your heater secondary does not have a center tap but is grounded instead through 2 resistors or a hum balance pot, I would not recommend using it for powering auxiliary circuitry as this will probably result in severe hum and buzz.
    Attached Files Attached Files

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    That is excellent flyingdutchman! Thank you for that info.

    A quick edit on my post: I do have a CT for the heater but it is elevated. Will that cause any problems?

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    Quote Originally Posted by chunkitup View Post
    A quick edit on my post: I do have a CT for the heater but it is elevated. Will that cause any problems?
    If by "elevated" you mean that the CT is not connected to ground, then yes, this may cause problems. The purpose of the center tap is to make the heater voltage balanced to ground, so as to minimize hum in low level preamp stages. Also, in order for the circuit I posted to work correctly, the heater secondary CT must be grounded.

    A little caveat regarding the circuit I posted: the input should be connected as close as possible to the power transformer. All rectifier circuits with capacitive loads draw current pulses at the peaks of the AC waveform, and you don't want these pulses traveling along the entire heater chain. Keep the input wires short, and twist them together neatly.

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    Well it is connected to ground but at about 50v to help with any hum. I am able to run my switching circuit with the elevated ground

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    Quote Originally Posted by chunkitup View Post
    Well it is connected to ground but at about 50v to help with any hum. I am able to run my switching circuit with the elevated ground
    Connecting the heater supply to a DC voltage above ground as far as I know doesn't mitigate hum, the true purpose of this trick is to reduce the DC voltage differential between cathode and heater if you have some valve with its cathode potential considerably above ground. Such situations may occur in certain circuits (totem pole or SRPP) used in HiFi preamps, where the cathode of one triode is connected to the anode of another. The cathode of the "upper" triode thus sits at about half the B+ voltage.
    It isn't really an issue in instrument preamps. A cathode follower may have its cathode quite a bit above ground but I've never seen any problems which would justify lifting the heater voltage.

    The circuit I posted won't work with such an "elevated" heater voltage, it needs the CT solidly connected to ground.

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    Old Timer J M Fahey's Avatar
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    Well it is connected to ground but at about 50v
    Then it is NOT connected to ground.

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    Suddenly I feel like I haven't learned anything in the 5 or 6 years since I started working with amps. I thought that it was the ground for the heater circuit. Perhaps just the return path.

    Edit: Feeling blue now that cold weather has returned and stupid because my self education leaves me relying on you smart people to help me out because i cant retain anything. Good ole enzo is hurting and he chimes in to help. Thanks bro. Good people!

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    Last edited by chunkitup; 10-23-2013 at 09:12 PM.

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    'Connecting the heater supply to a DC voltage above ground as far as I know doesn't mitigate hum'

    See Tech Info
    Q: Why do some amps connect the center tap of the filament winding, or the junction of the two resistors off the filament string, to the cathodes of the output tubes?
    A: In a cathode-biased amplifier, the cathode is at a positive voltage, somewhere around 10-40V with respect to ground. If you elevate the filament "reference" above the potential of the cathode by connecting the center tap to this point, you can effectively reduce the amount of hum coupled into the tube. This is because the filament is now positive with respect to the cathode, so the cathode doesn't attract electrons (i.e. hum) from the filament. This is a very inexpensive and easy method of reducing the hum in an amplifier without having to go to a DC filament supply. In a fixed bias amplifier, the output tube cathodes are usually at ground potential, so you have to add a voltage divider from the plate supply to generate the elevated filament reference. You can experiment with the voltage level to determine the value that best minimizes the hum. Be sure to bypass the junction of the resistors to ground with a suitable filter capacitor, or you may inject some buzz or noise into the amplifier from the power supply.

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    so the cathode doesn't attract electrons (i.e. hum) from the filament
    It sounds halfway plausible, but I have strong doubts about this.
    There aren't supposed to be any electrons going from the filament to the cathode, or vice versa, anyway. The filament in indirectly heated valves is supposed to be isolated from the cathode, and any leakage that will show up as hum on the usually relatively low impedance of the cathode would indicate a bad valve.

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    Why are we required to use the heater supply for this op amp? Op amps will run MUCH better on 15v rails, and if this is a Peavey amp, it probably already has such rails in place. Why screw around? Peavey uses 30v (15v x 2) for the reverb IC on just about every amp they ever made with a a reverb. That Delta Blues used a transistor for the FX loop drive, but the reverb was straight op amp.

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    The TI data sheet for the TL072 lists the common mode input range with +/- 15V rails as +/- 11V minimum, +15V to -12V typical. This is what happends when the negative limit is exceeded and the current source for the input diff amp is cutoff. The same thing can happen to a solid state power amp if you hit it with a big enough signal, like from a tube preamp.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    I tried elevated heaters on a few amps, but never got any better (nor worse) results than just grounding the center tap. I'd say try grounding it and see if there is any difference

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    'There aren't supposed to be any electrons going from the filament to the cathode, or vice versa'
    That doesn't prevent a heap of buzzing entering the signal path if the ground ref on the heaters is compromised.
    There have been previous posts on this topic (ie mitigating hum by dc elevation of heaters) on MEF, I'll have a search later if none pop up.
    Pete

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    That doesn't prevent a heap of buzzing entering the signal path if the ground ref on the heaters is compromised.
    Agreed, but this takes place mostly through capacitive and inductive coupling, happening outside the valve envelope.
    The point here is, a free floating power transformer secondary is bound to be dirty as it's capacitively coupled to the mains. Referencing it to ground tames it a bit.

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    Lifetime Member Enzo's Avatar
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    Grounding a floating 6v winding will reduce the hum a floating one contributes. However, that does nothing to solve H-K currents, which are real, but not always present. Those are two separate sources of hum, and each has its own cure. On a grounded CT heater, more than half the 6v waveform the cathode is more positive than the heater of a typical triode, typical meaning sitting at a volt or two positive. There are indeed loose electrons boiling off the heater, and if the cathode is more positive than the heater, they are attracted to it, and that is an AC current through that cathode - hum. I don;t deny there are also radiated and capacitively coupled hum sources as well. ALL of those hum sources exist.

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    Quote Originally Posted by Enzo View Post
    Why are we required to use the heater supply for this op amp? Op amps will run MUCH better on 15v rails, and if this is a Peavey amp, it probably already has such rails in place. Why screw around? Peavey uses 30v (15v x 2) for the reverb IC on just about every amp they ever made with a a reverb. That Delta Blues used a transistor for the FX loop drive, but the reverb was straight op amp.

    Heres the scenerio Enzo:
    I built a friend a sloclone with the +4db fx loop. He cant use any of his pedals with it and asked for a -10db loop. I put in a metro zero loss fx loop which colored the sound too much for his liking. He said he wished I could put in the fx loop from one of his PV amps(need to find out which one) because his effects worked perfect with it. The problem is the PT only has 720vac and 6.3vac secondaries. That is why I posted this thread to see if it was possible. I decided to try and make my own hi voltage fet loop and you can bet that I will be starting a thread here for help designing it.

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    Old Timer J M Fahey's Avatar
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    FWIW if you have around 450V DC available somewhere inside that chassis (I bet you do, think screen supply), you can easily lower them to 30V (same as +/-15V) and use a single supply Op Amp circuit.
    A typical TL072 (1 should be enough) "eats" 3.5mA tops , so a 120K 2W resistor will do just fine.
    Just add a 220 to 470uF x 50V cap and a 30V 1W zener in parallel with it and you are done.
    Just pad the "send" signal some extra 12 to 18 dB (1/4 to 1/8 X) and add same gain to the recovery stage and you will be fine.

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    Lowering 450V to 30V would be a waste of power. This is how I would solve the problem (with high voltage FETs):
    Mojotone Discrete Hi-Voltage Series Effects Loop for Vacuum Tube Amplifiers - Mojotone.com


    Mark

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    Quite some time ago I built an effect loop for a red-knob Fender Champ 12 using a common JFET (the BF245) cascoded with a high voltage video driver transistor (BF469). The amp's owner wanted the loop to be as transparent as possible, and was mighty pleased with the result.

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    Considering that LND150 is readily available and you have the high voltage source I wouldn't bother with any opamps and stuff.

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    Old Timer J M Fahey's Avatar
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    Waste of power? Compared to what?
    It's a tube amp, wasting power all over the place just to keep those funny metal tubes red hot so they work at all.
    The less than 2 W I suggested are comparable or less han other possible solutions.
    Care to share some schematic so we can calculate actual current/power consumption?
    Otherwise it's all just wild speculation.

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    Quote Originally Posted by MarkusBass View Post
    Lowering 450V to 30V would be a waste of power. This is how I would solve the problem (with high voltage FETs):
    Mojotone Discrete Hi-Voltage Series Effects Loop for Vacuum Tube Amplifiers - Mojotone.com
    It's rather absurd to be concerned about 2 Watts in a tube amp, and then offer a $70 USD solution. That Mojo loop probably consumes between 1 and 2 Watts.

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    The loop circuit is build using two LND150 which is less than $1 each. So the price is almost the same as the price of two opamps. You can either buy the circuit from Mojotone or build it on your own. I don't have the schematic but it wouldn't be difficult to draw one.

    Mark

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    Bent Member Chuck H's Avatar
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    Re: DC elevated filaments. There are plenty of circuits that do raise Vck above the filament voltage. In these cases elevating the filament string for hum reduction makes sense, and I've done it with good results. Another situation where elevating the filaments is useful would be when circuits with a high Vck, like cathode followers are used. Modern history has demonstrated that some new era tubes aren't as robust as those from the golden age WRT filament to cathode differential. And some circuits with cathode follower circuits have failed because of this. Elevating the filaments reduces that differential.

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    First my apologies for the necrobump. I'm currently trying to design the most compact loop board I can (sockets off board) that I can slot whenever I'm space limited or just use as an output buffer for preamps. I've kicked a few HV loop designs around but always got caught out by some weirdness somewhere so I thought I'd try out an opamp loop using a HV voltage clamp as a supply which is a pretty tried and tested approach. As ever, I like to sim stuff first if I can to get a ballpark idea of what is going on which leads me to a bit of a spice headscratcher.

    If I use a fixed +30V supply and a +15V rail for a virtual ground/VREF then the circuit behaves as expected and can swing up to 30Vpp before clipping which is good enough for me.

    When I try and use the voltage clamp/Zener regulator it goes to hell. When it's not connected to the opamp circuit I get the expected rails but when it is hooked it the +30V rail drops to +8.5V and the +15V rail drops to less than 1V

    Have I made a noob error that I can't see or is this just some spice oddness?

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    Weird!
    Perhaps c3 and c8 values are a lot higher value than necessary / beneficial.
    Maybe try c8 to 0V rather than Vref.

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    Quote Originally Posted by pdf64 View Post
    Weird!
    Perhaps c3 and c8 values are a lot higher value than necessary / beneficial.
    Maybe try c8 to 0V rather than Vref.
    I've just tried c3 & c8 at 100n and c8 to 0V and still the voltage rails are screwy!

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