Noob with a soldering iron

[g1]

  The center position of the switch is where it must be set when using an external footswitch.
See pg.5 of schematic.  They call it "off", but it looks to me that when set in centre, with no external footswitch, everything should work.

[g1]

  To clear up a couple more design/operation issues:
input 1 is for ch.A use only
input 2 is for ch.A or B
So that should explain some of your questions about your tests
(http://www.award-session.com/pdfs/Award-Session%20History0026.pdf).

[On a side note, it irks me that they have called the clean channel "B".  This is so non-standard, I have to keep reminding myself B=clean here   :grr]

Now, 2 issues you mentioned previously.  You said clean channel was quite weak and required adding in from dist. channel to get level.
  This does not seem right and is possibly a defect that has been overlooked.  Using ch2 input, switch set to clean, how far up do you have to bring the volume to get decent level (without adding anything from dist. channel) ?
  The other thing you mentioned was a bit of distortion at the trailing edge of the note as it fades.  Maybe a cross-over distortion issue.

[Jungle-Jim]

Thanks g1

To answer your question:
Yes the clean channel is not as loud as the OD channel - but that's more or less to be expected. The clean channel is reasonably loud, but you get a whole new level of noise (overdriven) if you crank the OD channel. I don't know if that's a problem per se.

Yes there *was* bad 'trailing edge' distortion on a guitar sound, but I think between changing all the pre-amp ICs to LM883N's and putting those 22pF caps across the op-amps which were missing, that problem was reduced by about half - in other words I can still hear some sort of glob of noise just following a note, but it's not quite as bad as it was.

Yes making input 2 (not 1) the main input jack for the instrument seems arse-about doesn't it.

Thanks for clearing up the design questions about this amp - it's always been a bit confusing. I have seen that article before.

This amp is flummoxing me - both pre-amp channels have some hiss/noise, and I just don't know how to track it down. And this hiss can't be 'just how this amp is' because if I bought a new amp and it had this sort of hiss I'd been returning it under warranty.

Jim

[Roly]

There are the three tracks which come into IC3a - from the Clean, OD Channel and Reverb, each with a resistor. I desoldered one leg of these resistors to disconnect these in this sequence...

Good diagnostic thinking.   :dbtu:

Is there a clue in that switch setting doing something a bit unexpected? Maybe not. What about that other idea mentioned about shielding the caps at the input stages.

Nah, just a draftsman being lazy.  The cap shielding was addressed to Phil with his layout.


Now we have the feedback caps in place I think we can eliminate HF instability due to the LM833's having such a wide bandwidth.

Given that we have now sorted out the IC misnumbering it might be an idea to revisit the progressive re-insertion of the op-amps working from the power amp input back towards the guitar inputs, measuring the hiss at each step. 

I'm still confounded that you are getting hiss with LM833's in a moderate gain circuit.  I've found them effectively silent and I'm dubious that what you are hearing is being generated in the IC's.

Perhaps if you could upload a few seconds of sound sample so we can hear the character of the noise?

{This one certainly proves the point that it is the subtle faults that are hardest to fix.}

[Jungle-Jim]

Thanks Roly
I'll do this test again tomorrow, then do the audio recording when I can over the next few days.

Given that we have now sorted out the IC misnumbering it might be an idea to revisit the progressive re-insertion of the op-amps working from the power amp input back towards the guitar inputs, measuring the hiss at each step.

So this must be the new sequence?...
(power off)
Insert IC3 (the output end of the preamp);
(power on)
Measure residual noise at the amp output;
(pwr off)
Insert IC1
(etc)
Then 2 then 4 (reverb section).

Perhaps if you could upload a few seconds of sound sample so we can hear the character of the noise?

Will do in the next few days - will take a little bit of setting up of recording gear.

Roly - if these tests yield nothing conclusive - are we at the limits of what we can diagnose without an oscilloscope? Would this be 15 minutes work running a probe around the circuit? The last tests, isolating these sections of the circuit, showed that both channels are generating hiss, albeit the OD channel being the bigger culprit. 

Jim

[Roly]

are we at the limits of what we can diagnose without an oscilloscope?

Well you can do the MacGyver with a length of wet string and an unbent paperclip, but a 'scope certainly would make life a lot easier since you are trying to look at a signal that is low-level AC which is much more 'scope territory than multimeter territory.  I don't know about "15 minutes" but the view of the problem would be less obscure.

There are freeware software CRO's that will turn your sound card into a sort-of oscilloscope that may be sufficient for this job (but I would put a 0.1uF cap in between the soundcard active Line In and your probe, just to give it a bit of protection - or drag out that old desktop 'puter you don't care about).

These are pretty hopeless for absolute measurements but just fine for relative measurements, and that is really what you are interested in.

As both channels seem to be acting the same I'd concentrate on the simple clean channel for the moment.

[J M Fahey]

are we at the limits of what we can diagnose without an oscilloscope?

Well you can do the MacGyver with a length of wet string and an unbent paperclip, but a 'scope certainly would make life a lot easier since you are trying to look at a signal that is low-level AC which is much more 'scope territory than multimeter territory.

There are freeware software CRO's that will turn your sound card into a sort-of oscilloscope that may be sufficient for this job (but I would put a 0.1uF cap in between the soundcard active Line In and your probe, just to give it a bit of protection - or drag out that old desktop 'puter you don't care about).

Agree.

Use any software scope which uses the 1/8"PC soundcard line in as input, but put bthis between the real world and that fragile input:



http://www.zeitnitz.eu/scope_en

[phatt]

As both channels seem to be acting the same I'd concentrate on the simple clean channel for the moment.

I'm with Roly, Disconnect ChA and Rev and work back through ChB until you find the issue.
With only Clean in circuit maybe flick that channel switch and see if that makes a difference as
somehow these 2 channels are interconnected after the first stage, that part along with those extra diodes on ChB has me stumped as to how that works. :loco
Phil.

[Jungle-Jim]

Many thanks again Roly and Juan

I've learned so much in the past month or two, and the adventure continues...

I'll go back to the actual amp in the next post, this one's about the software oscilloscope...

OK this is brilliant - I certainly have computers and sound cards lying around. How's the Linux version - called xoscope? That'd save me having to get a Windows PC up and running - or is www.zeitnitz.eu the least pain-in-the-a** option?

With your buffer circuit, Juan, are all those resistors 1/4w? And what are the diodes - 4148 or something bigger? I noticed that the xoscope guy had a fancy buffer which cleverly fits into a 5 1/4" disk blank, becoming a front-mounted fixture. http://xoscope.sourceforge.net/hardware/hardware.html

This being yet another thing I have never done, but would like to attempt... so if I did get this 'scope going, what tests would I perform with the probe?

For instance - would I work backwards from the point the pre-amp goes into the power amp, watching the shape of the wave, then keep going til I find the points where the hiss begins?

With any lucky the next test I do, which will be Roly's suggestion of pulling the pre-amp op-amps out, and replacing them in order, that might tell me something. I guess because the hiss happens on both the Clean and OD channels suggests the problem starts early in the chain of op-amps.

Thanks again,
Jim

[phatt]

Just be aware; (If I'm reading this right) Even when using the clean ChB and nothing plugged into Hot ChA any noise generated in the hot circuit could still bleed through to mixer section because it's output to mixer is still engaged. (more common to ground that point with a switch)

There is a fair chance the issue is in those middle stages of Hot ChA and even when not in use the hiss might be bleeding through. There is also that funny cross link after the first 2 input stages controlled by the switch.
Sw1 does not kill the hot ChA when the switch is set to ChB

Having tested the Clean ChB on my bench with the schematic shown it's not insane noisy so I'm inclined to think something else is going on so if you isolate that and Still get noise ,, then we have to find another way to fish. :loco
Phil.

[J M Fahey]

LINUX XSCOPE should work fine.
The one I linked was just one among many.

In fact it has a 15 day trial period or something, while I'm quite certain the Linux one is truly free.

As of the input buffer, that guy (like 99% today) comes from the Digital world, and 5V is as high a voltage as he'll ever meet, so he either buffers or preamplifies his signal (I guesss he expects an electret microphone driving it)  while we come from stone age tube electronics, with macho voltages 500V or worse, so we usually think attenuate!!!

The resistors can be 1/4W ones and the diodes any you have, from 1N914 or 1N4148 to 1N400x

The scope suggested is not sensitive enough to measure noise directly at every stage, but we don't actually need that.

You scope the speaker output, will clearly see noise and more important, what kind is is (real noise is called "grass" for good reason, you'll notice why) , while RF oscillation, which often sounds like noise, looks very different.

Then you will molest your amp, by tweaking controls, shorting to ground different key points, switching channels, etc. , until you see what kills or substantially lowers it.

[Jungle-Jim]

Hi all

The following is the results of some tests Roly had asked me to do in previous posts...

Test A  - testing mV output with ICs inserted in the order of 3,1,2,4...

"(power off)
Insert IC3 (the output end of the preamp);
(power on)
Measure residual noise at the amp output;
(pwr off)
Insert IC1
(etc)
Then 2 then 4 (reverb section)."

I put the DMM across the speakers, set to 200mVAC... All these tests were done with the 270k Res connecting the OD channel one-leg-unhooked. So just Clean ChB. To save myself having to make a table here, I've attached a spreadsheet below giving the mV results of various permutations of ICs being removed. I hope it's self-explanatory.

Test 2
On Jan 5th, Roly gave me these tests to try...

1. Select channel B, Ch A volume, overdrive, filter to zero.

Starting with the Fx Return shorted, speaker connected, and AC millivoltmeter clipped across the speaker.
Confirm you are still getting your 0.4mV reference noise level

This test came out as 0.7mV - I was getting 0.4mV before - don't know why the change. In this test the pre-amp hiss goes to almost nothing, revealing an underlying hum which I know would improve with a better power supply. I'll think about improving the power supply once I get rid of this f*cking hiss.

2. Remove short from Fx. (oops!)
Insert IC2, Ch B volume at zero.
This will give you the noise contribution of the IC2b stage only, the preamp output buffer (which has a gain of about x2).  Record.

I'm inserting IC3 now, because that's what we used to call IC2, the final IC in the chain. Result: 0.7mV (same as if the Return was shorted - but it sounds very different)

3. Center the Treble and Bass controls, short the wiper of the treble pot to ground with cliplead o.n.o, Ch B volume to max.
This will give you the cascaded noise contributions of IC2a and b (~x10 x2).  Record.

Again, IC2 is now IC3 - result - 91.8mV and a terrible moaning hiss. Take the ChB vol pot to zero and it's back to 0.7mV

4. Remove the short from the treble pot and place it across the 1M resistor connected to pin 3 of IC1a.  Insert IC1.  Set treble and bass to 12 o'clock/50%.
This will now include the noise contribution of IC1a (~x10 x10 x2).  Record.

This test wasn't affected by the IC's being wrongly labelled. Result - with the ChB Vol at 100% this test with the outside leg of the 1M grounded, comes out as 2.1mV. If you take the short off the 1M it's back to 1.8mV (that's with vol 100%).

5. Remove the short on the 1M resistor, and record open input noise level.

This will give us 0.7mV if the Chb vol is 0%, 1.8mV if the ChB vol is 100%, as per usual.

Are these tests and the spreadsheet showing anything?

Some observations:
* With no IC's at all, the output jumps to 8.1mV and there's a moaning hum.
* Once IC3 is in, there is this hiss floor as usual, even with all pots on 0.
* That once IC3 was in, and no matter if it was in on its own or the other ICs were added it gave the same readings - ChB vol at 0% = 0.7mV, ChB Vol at 100% = 1.8mm. And at 0% the hiss was at it's usual lowest level, and at 100% the hiss rose by the same amount in each instance.

Conclusion?
The hiss is with IC3, and it's there even with all pots at 0. If the problem's in IC3, the final IC in the chain, and IC1 doesn't lift the hiss floor, then that might explain why both ChA and ChB are affected by hiss - because IC1 and IC2 are not at fault, it's the last stage of the pre-amp.

If so - what tests can I start doing on IC3?

Phew this turning into a saga to rival War And Peace.

Thanks again for your patience.
Jim

[Roly]

Try sticking a 0.1uF (or what have you) directly across the IC power supply pins 4 and 8 on the copper side right under the IC3 chip, nice and close.

{Just a wild idea.}

Elsewise I'd say Phatt's suggestion of lowering the impedances around this stage may help.

Subtle problems can be tricky.

[Jungle-Jim]

Try sticking a 0.1uF (or what have you) directly across the IC power supply pins 4 and 8 on the copper side right under the IC3 chip, nice and close.

{Just a wild idea.}

Elsewise I'd say Phatt's suggestion of lowering the impedances around this stage may help.

Subtle problems can be tricky.

Thanks Roly
OK good idea I'll try the cap under IC3 pin 4 & 8. If this made a change it would certainly prompt an upgrade in the power supply.

But back to Phatt's suggestion about lowering the caps/res around what we now call IC3a, the very last op-amp before output. Bear in mind that I have already done the other experiment on IC3b (AKA the 2nd op-amp in the Clean channel) - where I increased the caps and res 100x - and it made no difference.

After all the confusion of the ICs being mislabelled, and before I start soldering the board again, could I just get you or Phatt to confirm again what this other mod was supposed to be?

Was it about dropping the res's 10x around IC3a? Currently V- or pin2 brings in the outputs of the three circuits - ChA, ChB and Reverb, via 270k, 560k and 470k res respectively. And across pin 1 (Vout) and pin 2 (V-) there's a 1M res, and I put in a 22pF ceramic on blank pads which had no cap (I really don't think this cap helped much if at all).

Thanks
Jim

[phatt]

Working on only the clean Ch B I've circled the two resistors to change in red.

With Reverb and ChA not in circuit this should make the clean quieter, If it does then I'm certain there is just simply to much gain especially after the tone pots as that is a hi z setup which can be a dog to keep quite. xP
I had a dog of a time with my PhAbbtone circuits until I learned about High Z and high gain are a bad combination.

This will just turn down the sensitivity of the IC that picks up the tone and then add the extra gain in the next stage, as can be seen the outcome is the same but should deliver less hiss. You share the gain around until you find the best compromise.
A buffer is good at isolating the high Z but more parts count.

Phil.