OT: Bass preamp clipping problem (with link corrected)

[rubberlips]

Okay guys, I threw the schematic on photobucket - that should work. And I started a new topic  for it

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I have a question about a bass preamp circuit that I'm having trouble with.
Here's the part of the schematic that's giving me problems
http://img.photobucket.com/albums/v680/rubberlips1/bass_preamp.jpg

It's clipping the top and bottom of the wave when I feed a sine wave in when you turn the treble control up on pin 6, much like it's hitting power rails, however the voltage is only 2V pp. The supply rails are +/-15V.

From what it looks like it's a slew rate problem. When I change the NE5534 to a TL070, it's not clipping anymore, but the wave does change shape (the bottom end seems okay, the top end is limited slightly, but it's still a sine wave). Maybe it's got more to do with the frequency I'm feeding in, after all it's supposed to be a bass amp.

Also, the pins 5&8 seem to be mixed up on the board and the schematic. Which is correct for the DC offset control?

One other thing about the offset control I'm not sure about. On the PCB, the 22K resistor goes to +15V, however on the schematic it's says it goes to signal ground? Which would be correct?? I'd take it the PCB would be correct??

There's a few boards for this bass preamp (preamp stage, equalizer and compressor), and I think it's distorting in a few spots, so I'm trying to weed out the problems one at a time

Any suggestions??

Cheers

Pete

[rubberlips]

bump...
Any ideas guys??

Or does anyone have any links to bass preamp schematics they found useful??

Cheers

Pete

[Rob Strand]

That sure looks like the preamp from the ETI1410 bass amp to me (from ~1984).  Many boards... It had the PA MOSFET power amp, graphic EQ, Limiter (which was crap) - is that it?  I can send you the complete article if you want it.   Design was OK except for the first stage preamp, which was inverting, and the limiter.  Tone control frequencies are  fairly nominal and up to the user, however, modern bass amps tend to have slightly higher treble control frequencies.

The null control for the NE5534 should be:
-  22k resistor to +V
-  pin 5 should only connect to the compensation cap.
  ( other side of comp cap to pin 8 )
-  null pot outers on pin 1 and pin 8

I don't remember any official errata for that project.  However, I do remember scribbling on the schematic because it had an error (not the one you have found).  So I'm not surprised you have found another error.  PCB errors are a different matter, those will obviously cause problems.

Since you can improve things by changing the opamp perhaps the NE5534 is dead?  By the way, with a different opamp you should completely remove the offset pot, 22k and comp. cap, these are totally wrong for other opamps - the NE5534 has funny pins for these functions.

I will check the eratta.

[rubberlips]

That sure looks like the preamp from the ETI1410 bass amp to me (from ~1984).  Many boards... It had the PA MOSFET power amp, graphic EQ, Limiter (which was crap) - is that it?  

Spot on there Rob! Built it quite a while back but could never get the darn thing to behave properly. The preamp clips, the EQ clips, and the limiter doesn't work at all, but I thought I'd work it out as I go. I changed the mosfet power amp with a trusty 300W power amp I built years ago, it's a work horse.

I can send you the complete article if you want it.  Design was OK except for the first stage preamp, which was inverting, and the limiter.  Tone control frequencies are  fairly nominal and up to the user, however, modern bass amps tend to have slightly higher treble control frequencies.  

Thanks, I got the article but I thought there might have been an errata sheet for the project  like they usually did for that mag!

The null control for the NE5534 should be:
-  22k resistor to +V
-  pin 5 should only connect to the compensation cap.
  ( other side of comp cap to pin 8 )
-  null pot outers on pin 1 and pin 8

So I'd gather the PCB's wrong, and the schematics correct for pin 5 & 8 as   pin 5 & 8 are swapped on the PCB compared witht he schematic

I don't remember any official errata for that project.  However, I do remember scribbling on the schematic because it had an error (not the one you have found).  So I'm not surprised you have found another error.  PCB errors are a different matter, those will obviously cause problems.  

What other errors were there?

Since you can improve things by changing the opamp perhaps the NE5534 is dead?  By the way, with a different opamp you should completely remove the offset pot, 22k and comp. cap, these are totally wrong for other opamps - the NE5534 has funny pins for these functions.

Yeah, I think I'll have to put teh NE5534 back in, seems there's some sort of rumbling coming out of the preamp board now. Doesn't have any DC coming out, small AC voltage (couple of mV), but there seems to be some high frequency (about 140kHz) coming out - the power amp really doesn't like it  

I will check the eratta.

Cheers

Pete

[Rob Strand]

I thought I'd work it out as I go

You should be able to get it working and sounding half decent - it's not that much different to many of the pro bass amps really.

errata sheet for the project like they usually did for that mag!

I didn't remember any.  I checked the errata for that year and the another errata covering all ETI projects and that projecvt has no errata.  The only other hope is it's in the second part of the article (somewhere).

So I'd gather the PCB's wrong

Had a quick look, looks wrong to me too - obviously needs a track cut etc.

What other errors were there?

From what I can see it's on the limiter.  On limiter schematic (page 68) I've scribbled on the two pins at the bottom of the NE570 chip IC9. I've got the left most one marked as pin 4 going to ground, and the right most one marked pin 2 going to the 2M2 resistor R49. I've also removed the join between the two pins.  Probably schematic only but I'd check the PCB to be sure.

Also on the limiter I've got IC10a marked with the + input as pin 11 and the - input as pin 10.  Can't see what it was previously. Again this is the schematic.  Don't know about the PCB.

I recommend increasing C52 on the limiter.  I've has these NE570 limiters burst into oscillation.  Can't remember the value but 47pF might do.

The main beef I had with these limiter is there is a large DC feedthrough on the output when you drive them into limiting - it's caused by the NE570.  It sounds like bad clipping.

PS: I don't know why the EQ would clip, it looks pretty conventional to me.     Sometimes you can get oscillations on these EQ's.  Try increasing C24 and C25.

If the bass and treble control stage oscillates try putting a cap, say 100pf (perhaps play around), from the output to the -input.

Given the PCB(s) is in question I'd be checking as much as I could bare to check - painful as is it to check.

If everythging is OK all stages should be able to swing quite close to the rails. If not check the PCB and perhaps check that the electrolytic caps, or even the pots, haven't gone.

Good luck!

[rubberlips]

You should be able to get it working and sounding half decent - it's not that much different to many of the pro bass amps really.

That's what I thought, looked fairly straight forward from the schematic, except for the limiter board

I didn't remember any.  I checked the errata for that year and the another errata covering all ETI projects and that projecvt has no errata.  The only other hope is it's in the second part of the article (somewhere).

No nothing on my copy of the second part either.

Had a quick look, looks wrong to me too - obviously needs a track cut etc.

I'll slice it up tomorrow and see how it goes

From what I can see it's on the limiter.  On limiter schematic (page 68) I've scribbled on the two pins at the bottom of the NE570 chip IC9. I've got the left most one marked as pin 4 going to ground, and the right most one marked pin 2 going to the 2M2 resistor R49. I've also removed the join between the two pins.  Probably schematic only but I'd check the PCB to be sure.

Also on the limiter I've got IC10a marked with the + input as pin 11 and the - input as pin 10.  Can't see what it was previously. Again this is the schematic.  Don't know about the PCB.

I'll check them both

I recommend increasing C52 on the limiter.  I've has these NE570 limiters burst into oscillation.  Can't remember the value but 47pF might do.

The main beef I had with these limiter is there is a large DC feedthrough on the output when you drive them into limiting - it's caused by the NE570.  It sounds like bad clipping.

I've actually got a NE571 in there. I checked the data sheet and looked like the pinouts were the same

PS: I don't know why the EQ would clip, it looks pretty conventional to me.     Sometimes you can get oscillations on these EQ's.  Try increasing C24 and C25.  

That's what I thought, I've checked a few different schematics (as well as the preamp treble and bass controls) and they're very similar to other guitar preamps, the caps and resistors are a bit different but essentially the same.

If the bass and treble control stage oscillates try putting a cap, say 100pf (perhaps play around), from the output to the -input.

Of the tone section (betweenoutput of IC4 and output of the board?

Given the PCB(s) is in question I'd be checking as much as I could bare to check - painful as is it to check.

well, the preamp board did have a few tracks missing, which I fixed and buzzed out the rest, that's how I found some of the IC pins weren't the same as the schematic. I'll check the EQ board and see if there's something weird with that one too. Been a while since I worked on it.

If everythging is OK all stages should be able to swing quite close to the rails. If not check the PCB and perhaps check that the electrolytic caps, or even the pots, haven't gone.

That's what I thought was strange because it actually clips at about 2V pp, nowhere near to +/- 15V supply. The preamp parts probably got something to do with the pins being wrong on IC5.

I thought, find out if the schematics right first, and if I can't work out what's wrong with the PCB's, it's just as quick and easy to vero board them up again.

Cheers

Pete

[Rob Strand]

except for the limiter board

The limiter board is straight out of the Philips applications notes for the NE570/NE571.  I've never liked it.   I have a vague recollection of seeing it used in a commecial device - but I'm not quite sure.

IMHO the feedthrough problem makes the NE570/NE571, and for that matter the NE572, problematic for hard limiters.  The feedthough sounds as bad as the clipping.  I had one design which was OK.

NE571 in there. I checked the data sheet

They are the same device except the NE570 is selected for particular tolerances - in practice I have found no differences.

Of the tone section (betweenoutput of IC4 and output of the board?

Between pins 2 and 6 of IC5.

That's what I thought was strange because it actually clips at about 2V pp

Things should look better after the IC5 fix.

quick and easy to vero board them up again

I wouldn't go that far.  It's probably easier to work out what's wrong with the PCB (if anything).   Those old ETI boards aren't to cramped so they aren't too bad to work on.

I suppose on thing to check is that all the grounds and supply rails are correct between the boards - wouldn't be hard to miss on of those.

[rubberlips]

The limiter board is straight out of the Philips applications notes for the NE570/NE571.  I've never liked it.   I have a vague recollection of seeing it used in a commecial device - but I'm not quite sure.

IMHO the feedthrough problem makes the NE570/NE571, and for that matter the NE572, problematic for hard limiters.  The feedthough sounds as bad as the clipping.  I had one design which was OK.

What was that??

That's what I thought was strange because it actually clips at about 2V pp
Things should look better after the IC5 fix.

You were right about the IC5 with the DC offset control. I rechecked the pins and there was a track that goes to pin 5 instead of 8, so I cut it and jumped it to pin 8. Bit easier to bias the opamp now.

Okay I worked out the clipping problem - it was me. :oops:  I was looking at the wave on the computer and when it got up to 2V it was clipping. Should have got that one before. Either the sound card was clipping or the amp box before the sound card was clipping. Put the gain down on the cro amp box from 1 to 0.1 and all was well...boofhead. The preamp doesn't seem to go anywheer near the supply rails though - plenty of head room, but the EQ does.

There's still some clipping from what I can hear, but I think it's more to do with the quad box I'm using which can't really handle the power. It's not really rated for 300W. I'll try it on the Celestian in the shed when I get a chance. Although I have actually got more sound level out of the power amp running the bass through a mixer preamp (EAM sept 85 and also the ETI467) and it actually sounded better than this supposed bass preamp. I'll play with it a bit more and see what I can get out of it, otherwise I'll probably just use the EQ section and build another type of preamp I can't hear what I like.

I wouldn't go that far.  It's probably easier to work out what's wrong with the PCB (if anything).   Those old ETI boards aren't to cramped so they aren't too bad to work on.

Except if you heat the pads too many times and they come off. But I've had commercial boards do that too.

I suppose on thing to check is that all the grounds and supply rails are correct between the boards - wouldn't be hard to miss on of those.

First thing I did, all voltages correct

I think the power supplies a bit noisy. To make the +/-15V, it comes out of the transformer goes through +/- 2200uf caps then 100nF caps, then input of the regulators, then 1uF caps. I  had 22uf caps on both sides of the regulator before and thought I'd try something smaller if it was a high freq problem, but it didn't make much difference.
I'm not sure if it is the Power supply, maybe it's a ground loop. I actually ran the preamp off another portable +/-12V power supply and it was a bit quieter, but I can't seem to find any ground loops.

(I actually had to take the power supply apart, I was checking voltages and shorted the bloody negative regulator - blew the rect bridge, the regulator and the input cap...nice work) :shock So spent most of the afternoon rebuilding it.

I worked out what the problem with the EQ was when I put it into the power amp. The output of the EQ and the input of the power amp didn't have a cap which was putting some DC on the input of the power amp, so I chucked in a 10uF cap which seemed to do the job. The EQ section seems to have enough output. It actually swings almost to the supply rails.

Now onto the limiter board....I'll check what you recommended last time about pins 2 and 4 and have a look at the datasheet. If when I get it going it still sounds shit, I'll throw in a compressor with an LED/LDR. A mate of mine put one in when he made his bass amp and reckons it does the job for compression. Always thought it was funny - the amp was called Godzilla - twin 1500W power amp modules he got from somewhere. It would have blue mountains away and his three sisters

Cheers

Pete

[Rob Strand]

Except if you heat the pads too many times and they come off.

Agreed, those boards aren't that great.

I'm not sure if it is the Power supply, maybe it's a ground loop. I actually ran the preamp off another portable +/-12V power supply and it was a bit quieter, but I can't seem to find any ground loops.

If it's hum, I doubt hum is getting through the regulators.  Most likely grounding related.   It's not easy to pin down the problem on those multiboard layouts.

I worked out what the problem with the EQ

Cool.

Sounds like the whole thing is almost in a useable state now.

Now onto the limiter board..

I suspect you should be able to get it working - as for sounding good I honestly don't think it's worth the trouble.

twin 1500W power amp modules

That's an evil amount of power..... also an evil cost in speakers to connect to it!

[rubberlips]

Sounds like the whole thing is almost in a useable state now.

pretty much

Now onto the limiter board..
I suspect you should be able to get it working - as for sounding good I honestly don't think it's worth the trouble.

Well, it's sort of working, had input wires to the board around the wrong way. However the limit LED was one and all of sudden it went off and the board started working. I'm always a bit sus about electronics that all of a sudden start working.

Still a bit sketchy on the hum. I discounted all of the power and input/outputs to the other boards and only had the preamp board in and it's noisy. Uness it's placement of the power supply board - it's sitting pretty close to a bloody big transformer - but even moving it doesn't seem to make much difference.

You're right on the design of the limited - it hard limits and with a bass  you can't get the volume out because it limits the signal to 1V - no where near enough to run the power amp up to max.

No I think the time has come - piss the preamp and the compressor board off and build another! I might build up this circuit http://www.albertkreuzer.com/pics/el/pre/schem/preamp_sch21.gif
Uses FETs and is not overly complicated - as an amp should be

Or maybe this one
http://sound.westhost.com/project27.htm

You haven't seen any good bass circuits out there??

Cheers

Pete

[Rob Strand]

You haven't seen any good bass circuits out there??

It depends if you are into tube sound or solid state.

Solid state bass preamps all follow similar designs, in fact not much different to the one you have.  The only addition are those voicing/contour switches/controls which enable a predefined EQ contour - very much a personal preference and playing style thing, you like it or you don't.  A very effective EQ contour is to build a Fender tone stack with trim pots and adjust it to taste with all the EQ controls flat - perhaps tinker with the cap values to taste as well.  Once you ball park that you can tweak the tone with the Bass/Treble controls and/or graphic EQ you already have.

The on-line ones that come to mind are the later Fender ones and the Yorkville ones.   As you will see there isn't much difference between yours and the pro ones, and the pro ones follow a similar format.   You should tinker with the bass and treble cap value on the tone control.

[rubberlips]

It depends if you are into tube sound or solid state.

Definately tube for guitar - bass I can't hear that much difference because it's mostly clean sounds - distortions a different animal

Solid state bass preamps all follow similar designs, in fact not much different to the one you have. The only addition are those voicing/contour switches/controls which enable a predefined EQ contour - very much a personal preference and playing style thing, you like it or you don't. A very effective EQ contour is to build a Fender tone stack with trim pots and adjust it to taste with all the EQ controls flat - perhaps tinker with the cap values to taste as well. Once you ball park that you can tweak the tone with the Bass/Treble controls and/or graphic EQ you already have.

I'll look into that. I've been downloading a few comercial amps to have a bit of a look at different designs. I think the design I have was probabably alright for the 150W module it was intended but lack when you want a bit more out of it

The on-line ones that come to mind are the later Fender ones and the Yorkville ones. As you will see there isn't much difference between yours and the pro ones, and the pro ones follow a similar format. You should tinker with the bass and treble cap value on the tone control.

Rob, no model numbers come to mind do they - so I can check out their schematics??

I think I'll just keep the power amp seperate - it's always been a work horse - and try a few different preamps. That way I can also use it as a PA when needed as well. Jusr plug in what's needed and jam!

Cheers

Pete

[Rob Strand]

think the design I have was probabably alright for the 150W module it was intended but lack when you want a bit more out of it

Like your preamp, most commercial preamps are opamp based and clean.  That means the preamp doesn't have that much of an impact on the volume, it's pretty much determined by the power amp hitting the rails.  Having said that compression, an limiting *can* help increase the volume.  Also inappropriate EQ frequencies can stress the dynamic range yet produce little audible change.  You can get a bit more output by using a high-pass filter to cut-out the deep bass - some of the higher power commercial amps use this method (it also reduces stress on the speakers).

Rob, no model numbers come to mind do they - so I can check out their schematics??

Not really, they all follow a similar structure just with minor additions and deviations.  Ponder over the bassman units for example, but all are in the same vein - including the Yorkville stuff.

http://www.fender.com/support/amp_schematics/

(I thinks there's also a Fender BXR300R on the cool schematics site.)

For the high-pass filter see if you can find Walter Hartley's trace of an Eden amplifier front end.  Off hand, another example is the the Musicman bass amplifiers (IIRC the models have B suffix).

If I were you I would play around with the preamp you have.  Change the bass and treble caps to suite your taste.  Add a contour for experiment.  With clean stuff the sound is all in the EQ.  To help increase output: Add a high pass filter for experiment.   Add a good compressor.

[rubberlips]

Rob,

I think there is some sort of ground loop problem there - so I'll seperate the power amp as a single unit. The preamp I  think I'll splice in the EQ with the PA preamp I have that gives plently of clean boost with a bit of overdrive. Always sounded pretty good to my ears but didn't have a good EQ on it. Just have to find a good compressor.

I'm also going to solder up the JFET pre-amp I found - shouldn't take too long to knock up - most of the time is etching the board!
http://www.albertkreuzer.com/pics/el/pre/schem/preamp_sch21.gif
But I might leave a bit out of that one. Let you know how it sounds when I finish it

Thanks for th help Rob

Pete