SVT-2PRO Bias Problem: LEDs stay red on one side

[bob p]

I've run across 2 styles of SVP, the good one, and the very lousy one. Lousy = you can hear the buzz from the filament rectifier in the audio output. No amount of fiddling relieves this. What a shame. Another "improvement" that's just a big step backwards. If anyone decides to buy one, have a listen first. If you hear that telltale buzz, keep your cash in your wallet.

just to be sure, do you really mean SVP, as in the rackmount preamp, or did you mean to say SVT? I ask this because the SVP hadn't yet been mentioned in this thread, and some models of SVP do have a rep for being hissy.

[Leo_Gnardo]

just to be sure, do you really mean SVP, as in the rackmount preamp, or did you mean to say SVT? I ask this because the SVP hadn't yet been mentioned in this thread, and some models of SVP do have a rep for being hissy.

Our new correspondent hiwattdoctor from France mentioned the SVP preamp. Didn't mean to divert nor hijack but I thought I'd give it a mention in its good and not so good versions for anyone who might be tempted to get one. Another example of LOUD's backward march in technology kwality. I worked on one of the good ones recently and it was exactly what it was supposed to be. And I got one from LOUD Ameg about 10 years ago as a road backup, and it sounded like a 120 Hz and all the overtones beehive. LOUD Ampeg should have left a good design alone.

Back to the program already in progress.

[bob p]

if this 2-Pro deal falls through i might end up going with a preamp and some high powered rackable SS amps that i've got sitting around. so which are the good SVP and which are the bad ones?

[Leo_Gnardo]

if this 2-Pro deal falls through i might end up going with a preamp and some high powered rackable SS amps that i've got sitting around. so which are the good SVP and which are the bad ones?

I wish I had a fast straightforward answer for you.

What the LOUD Ampeg product designations are I must admit I'm not 100%. There's a SVP PRO that has a couple of users mentioning hum/buzz in reviews on Harmony Central and TalkBass. Also SVP CL - some users complain about insufficient gain on the TalkBass reviews. This may be the case, but power amp sensitivity varies from one model to another and perhaps the SVP users aren't taking that into consideration. Also there's plenty of pre's that have too much gain, so if a player is used to turning the master up to 5 on one and that's enough to hit the amp, then another pre he has to turn the master up to 10 to do the same, the interpretation is the second pre is lacking somehow. Although it may in every other respect be perfectly good.

One interesting control is "drive". It varies the DC supply to some of the plate circuits. Dial it up for higher voltage and get a cleaner crisper tone. Dial it down and the tone gets a little sloppy and funky simulating an old worn out SVT.

Hard to buy gear at a distance. I'd say have a listen to any SVP you can get your hands on. If you hear an annoying buzz, don't buy it. And keep in mind, some distance between the pre & power amp in the rack can make a big difference in the amount of hum/buzz induced into the pre by its proximity to the power amp's power transformer. (Or any other transformer or device that "throws" an AC field.) Also ground loops through the rack rail can raise hob with hum. Plus the usual ground loop through the AC line - lift ground on the pre or power amp and that may cut loop hum. If you do everything you reasonably can and hum/buzz persist then try a different preamp.

The buzzy SVP pre's I ran across had a rectifier bridge and hi value capacitor to run some, not all, of the tubes' filaments on DC. If I disabled that bridge and connected the tube filaments to a clean DC supply - no buzz. Nobody wanted to pay to do the fix so they were left as the factory built them. One of the reviews stated that his SVP ran a lot better after the factory tech had a go at it, others got some relief by trying different tubes. I wasn't able to get any improvement by swapping tubes and got very discouraged. I figured the rectifier switching noise was imposed on the filament line and showed up as noise in the preamp tubes that weren't DC powered.

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Good to see you mention arbitrage. Wow that's a high concept but it really does happen in equipment deals everyday as well as so many other exchanges.

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And economics, FWIW within the first week of engineering courses our professor made it clear that engineering solutions involved economics as well as design. If someone orders a dog house design, you can't propose a pyramid 400 feet high, or even 4 feet high out of stone block and expect them to like paying the bill. "But it'll last forever!" Not necessary for a doghouse. Now there are moulded plastic "dogloo" models that will last near to forever. Nobody thought of that in the early 70's. Somebody was "thinking outside the box."

I wanted to be involved in aircraft engineering. It was apparent that for the first 3 years all engineers took the same courses, and in their senior year took courses that defined their specialties. "After all the equations really are all the same, the names of the variables are all that's really different." That was the excuse given in the cookie cutter days of the early 70's. And my advisor was a crusty old git who did his best to discourage me. Finding that electronics and circuits courses were available to physics majors I went there instead. And my physics advisor let me take any EE courses I wanted. Thank you Dr. Eppenstein!

Year after I left a new Aero E professor joined the staff. I guarantee you've heard his guitar playing. John Tichy ( from Commander Cody and the Lost Planet Airmen). Things could'a been different if I just showed up later... And his son Graham is a heck of a guitar player too. And one of my customers. Funny the way things work out.

Thanks for putting up with my yarn spinning.

[bob p]

I fancy myself an arbitrageur.

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I had read the stories on talkbass about the SVP. I ran into a problem, though, that TB tends to be an end-users' forum and the people there don't use the terms that we use to define amp problems, so it's hard for me to read the posts there with an analytical eye. To someone like you or me, hum, buzz, hiss and noise don't necessarily mean the same thing. Lots of people claim the SVP are "hissy", but they never define what "hiss" means to them, but I don't think they're talking about thermal or shot noise. Maybe by "hiss" they mean 120 cycle hum. Or maybe they do mean noise. It's really hard to read their reviews and know exactly what they're talking about.

Regarding "gaininess" or lack thereof: I have read peoples' claims that the SVP is designed with an output that is below the industry standard 1.5V. I don't remember the number off-hand, but they said it's significantly below 1.5V. So people claim that the SVP doesn't t drive amps to full output unless they're Ampeg amps that are designed with to have a sensitivity of 0.775V. I've read about a modification to change the output that involves swapping out maybe 10 resistors or so. That isn't a deal-breaker for me -- it'd be easy enough for me to re-scale the input sensitivity on my amps. Of course, I'm not even convinced that the sensitivity is even mismatched -- isn't 0.775V the standard for a balanced output? This has me wondering if the problem is that people are hooking up half of a balanced output to an unbalanced input, losing 6dB along the way, and then complaining that the preamp has anemic output! If that's the case, then the problem isn't in the preamp, it's in the user!

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Great stories you've got there.

[Leo_Gnardo]

TB tends to be an end-users' forum and the people there don't use the terms that we use to define amp problems, so it's hard for me to read the posts there with an analytical eye. To someone like you or me, hum, buzz, hiss and noise don't necessarily mean the same thing. Lots of people claim the SVP are "hissy", but they never define what "hiss" means to them, but I don't think they're talking about thermal or shot noise. Maybe by "hiss" they mean 120 cycle hum. Or maybe they do mean noise. It's really hard to read their reviews and know exactly what they're talking about.

Regarding "gaininess" or lack thereof: I have read peoples' claims that the SVP is designed with an output that is below the industry standard 1.5V. I don't remember the number off-hand, but they said it's significantly below 1.5V. So people claim that the SVP doesn't t drive amps to full output unless they're Ampeg amps that are designed with to have a sensitivity of 0.775V. I've read about a modification to change the output that involves swapping out maybe 10 resistors or so. That isn't a deal-breaker for me -- it'd be easy enough for me to re-scale the input sensitivity on my amps. Of course, I'm not even convinced that the sensitivity is even mismatched -- isn't 0.775V the standard for a balanced output? This has me wondering if the problem is that people are hooking up half of a balanced output to an unbalanced input, losing 6dB along the way, and then complaining that the preamp has anemic output! If that's the case, then the problem isn't in the preamp, it's in the user!

As it often is.

Yes lots of times the musos swap terms for the various types of noise. And who knows what they really sense on the other side of their cases of tinnitus. I would hope "hiss" is the usual shooosh background - thermal noise plus similar noise artifacts. Buzz, just what it sounds like, the higher harmonics of 120 Hz from rectification and leakage of its energy into audio circuits. Hum, lower harmonics of same. Not often we really hear much real 60 Hz. For those in 50 Hz served parts of the world, swap 100 Hz for 120.

One could use a transformer (horrors!) to bump an insufficiently large signal up in amplitude to hit a power amp's input hard enough to drive it to clip. And IIRC the SVT (original) power amp could be run to clip with a 0.25V signal. And your Phase 400 - half a volt. Thar's a whole lotta gain. Some other power amps require 2V input to get to clipping. There's a lot of variation.

Yes some of the folks detailing their experiences may well be "throwing away signal" due to simply using the wrong kind of connectors. If it ain't done with an ordinary quarter inch patch cord it's just not done.

I'm gonna get some shuteye then see if I can retrieve some info on the "good" SVP I worked on about a year or 2 ago. Patientez SVP...

[bob p]

Well, it looks like I'm going to pass on the SVT-2 Pro. The seller finally responded with some better photos, which show that the amp looks like it was dropped on more than one occasion when it wasn't in a rack -- badly enough to put major deformities in the thick faceplate at the front/bottom, and on the top of the cage on the backside. The faceplate has a long curved dent in it that's bad enough that I'm convinced that the preamp PCB, and maybe one of the other PCB is going to have problems. Broken PCB are one thing that I'm not willing to deal with, so I'm out. thanks to everyone for their help.

Now Leo, about those SVP preamps ...

[Leo_Gnardo]

Now Leo, about those SVP preamps ...

It was an SVP PRO that last passed inspection here as the "type to have" but as always YMMV. Again, listen before you commit to paying if at all possible, if you find one for sale.

It's handy to have that graphic if you have an instrument or speaker that really has to be doctored up to sound good. Or to design "special" tones.

And of course you could always build your own and design the eq and output that makes sense for you.

Got an Alembic F2B, that suffices for me. You could build a one channel version (it's a Fender stock BF preamp used in practically every amp from Princeton to Twin with the exception of Super Reverb.) easy, add a 12AU7 or similar to drive an output transformer, then use that to skip over ground loops forever more. Put in an FX loop so you can patch in the EQ and/or other FX of your choice.

[hiwattdoctor]

yes it s a svp preamp with 5 tubes in it , i have just mods the xlr output to have the loop send signal in, resolderer the transformer and glue it with epoxy , fit it with JJ TESLA tube and connect it to the power amp of a concertbass sunn with a 2x15 box, the sound is very good never have a problem with this stuff , maybe in europe we don t have the same transformer ( 230vac 50 hz) no hum or buzz works very very well , if i can i purchase another one .....

[pacAir]

Just some additional info that may be of value to you if looking for an Ampeg SVP-PRO preamp:

I ran into some interesting tidbits regarding the Ampeg SVP-PRO bass preamp. There appears to have been at least 3 distinct generations of this product.

The Ampeg SVP-PRO "blackface" (introduced in 1990, with black faceplate, panels and chassis) had an active D.I. output with level control. There was no output transformer, just an op-amp IC. This version had a power transformer that was mounted with the exposed core oriented left and right.

This design was apparently carried over into the 2-tone (Black faceplate with Gray panels & chassis) "facelift" in 1995 except there was a change in circuit board layout which rotated the power transformer 90 degrees such that the core pointed front and back. It is assumed that this change lowered hum pickup caused by the magnetic field of the older orientation of the power transformer. (Note:These PC boards were typically white in color.)

Fast forward to 1999, a new version is introduced (still 2-tone aesthetics) and the only external difference appears to be that the D.I. XLR connector moved to the right side of the "Tuner Out" jack and lost its level control. Inside, the D.I. circuit was changed to that found in most modern Ampeg SVT-PRO heads... a passive transformer-coupled design. (Note:These PC boards were typically green in color.)

Also, on some post-1999 units a toroidal filter element was added to the AC input circuit (presumably to filter out line noise injection). The transformer-coupled D.I. output provides better electrical isolation and potentially less hiss because it is not an active op-amp with gain.

The best way to tell an early from later model 2-tone unit is the location of the D.I. XLR connector and the presence or absence of a D.I. output level control on the rear panel.

Most of this info was derived from internet sources (so it must be true) and evaluating SVP-PRO photos of all 3 variants. The SVP-PRO was discontinued in 2007. I have not had all 3 variants in-hand for inspection or testing.

Corrections or comments welcome.


Steve
Pacific Music Services