TS808 issue

[metamatic]

Hi
i've just finished my first project, a TS808 (tonepad schematics).
It seems to work properly, but it has a very low amount of distortion...
I don't have a stock TS808 to make a comparison, but i'm a bit confused about how it is supposed to sound. Youtube demos don't help, as i can't actually catch the sound as most of the times it is over-compressed or mic-clipped, but mine really seems to have just a little bit of distortion even when drive is full on, although it is really dinamic and fat sounding. I'm playing a stratocaster with Di Marzio virtual vintage '54 through a fender hot rod deluxe.
I thought there was some problem with diodes, but multimeter shows the right voltage drop..
Do you  think it is actually faulty (and if so could you suggest any test) or it is just the way it is supposed to sound?
thanks a lot, greetings

[flo]

With "gain" full open, it should have quite a lot of distortion. Not "metal" like but still quite some...
There are various places on the internet where soundclips can be found.

Perhaps you can do measurements and post voltages and such as requested by:
http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[metamatic]

Thanks, that's what i supposed it should sound like... so  something may be definitely wrong, as with drive cranked it makes just some light overdrive
I'll post some measurement later.
Do you think IC, transistor and diodes would be enough to start?
regards

[metamatic]

Now that i think about that i used polarized tantalium caps and i'm not shure i placed them the right way  .... could a reversed polarization cause the drive to be so low? (i cant'actually check now couse i don't have the board here...)
thanks.

[metamatic]

hi
just got some measures. I measured everithing with drive cranked, volume at 12 o clock, tone almost fully opened.
Refer to this schematic:
http://diy.musikding.de/images/stories/screamer/screamerschalt.pdf

IC1 ( JRC4558D from dotted pin, counterclockwise):
P1: 4.55 V
P2: 4.72 V
P3: 4.47 V
P4: 0 V
P5: 4.55 V
P6: 4.55 V
P7: 4.55 V
P8: 9.18 V

Q1:
C: 9.18 V
B: 2.62 V
E: 3.03 V

Q2:
C: 9.18V
B: 2.61V
E: 3.0 V

D1:
C: 4.55 V
A: 4.72 V

D2:
C: 4.72 V
A: 4.55 V

D3:
C: 4.55 C
A: 4.72 V

i used polarized tantalum caps, which are  correctly placed though.
I recorded a brief clip just to have an idea of how it sounds (same settings as measurements). Recorded with a strat with dimarzio virtual vintage '54 (center pos) and fender hot rod deluxe and sure sm58 mic... sorry for poor recording/playing/out of tune guitar.
http://digidownload.libero.it/abstract_toolkit/ts808.mp3

thanks a lot
regards

[flo]

The voltages on the IC look good.

The voltages on the Base of Q1 and Q2 seem to be too low. They should be 0.6V higher than the voltage on the Emitter. But this could be due to the internal resistance of your voltage measuring device.

Can you check with an audio probe how the signal sounds at the Base and at the Emitter of Q1 and Q2:
http://www.diystompboxes.com/pedals/debug.html

Actually, the soundclip doesn't sound bad but indeed if this is at max drive, the amount of distortion seems too little.

[Paul Marossy]

Did you check for any solder bridges? Sometimes a tiny little piece of solder or component lead slightly touching something connected to ground can whack the amount of gain you can get. I've had it happen to me many times.

[metamatic]

Hi
actually i get a lot  of gain (volume boost), but it lacks of distortion, it's quite clean.. Anyway i'll check for unwanted joints
thanks

[metamatic]

i tested the board with an audio probe.
On Q1 the level is quite low and seems not distorted at all, but i could be wrong (anyway should be the first stage, so it may be ok). Doesn't seem be that difference in amplitude from base to emitter
On Q2 i get a higher level, maybe same as output and distorted (shoud be the output stage) but again the signal doesn't seem to get that big boost moving from base to emitter.
I suspect for some reason the IC doesn't squeeze the signal the right way and that's the why the signal stays clean, but can't figure out the reason... are the diodes voltage ok?
any idea?
thanks for your support and regards.

[metamatic]

Any comment about the sound clip i recorded?
http://digidownload.libero.it/abstract_toolkit/ts808.mp3
how do you see the amount of distortion it gets?
regards

[earthtonesaudio]

The voltages on the Base of Q1 and Q2 seem to be too low. They should be 0.6V higher than the voltage on the Emitter. But this could be due to the internal resistance of your voltage measuring device.

I agree with this.  Are you sure the transistors are the right type/pinout/installed correctly?

[metamatic]

gotcha!
As i'm dumb i didn't realize i was using 47k Ohms instead of 4.7k to set the drive in the op amp ....
replaced with a 4.2k resistor (didn't have a 4.7) and now the drive is definitely increased (still not as much as a fuzz anyway but think it is how it is supposed to sound like) and it also sounds much fuller indeed..
thanks for all your help and replies
regards

[liquids]

I suspect it's something before the IC.  Even if not, you have to verify that all is well with the transistor buffer stage that comes before the IC stages, first things first.

If you are running right into the effect and then from the effect to the amp, than barring any oddities, the sound at the base and emitter of Q1 should be about the same volume as your bypassed signal. It should likewise be clean, not distorted, as you stated.

If the volume here is significantly lower than your bypass signal, something is wrong--stop right there.  Check and confirm the values of R1 and R3.  Make sure you don't have any solder bridges etc.  Post a close up of your board.

Edit: looks like you fixed it.  Nevermind.   

Note that I hate that kind of haphazard 'noisless biasing' on the base of Q1 and Q2, classic or not.  It's a great concept but not adjusted for real world functionality..a good buffer stage would bias the base at or somewhat above 1/2(+V).  In this exact case and power supply, above 4.7v....closer to 5.5v is ideal here, actually.   I'd eventually change R17 to a smaller value (try 4.7k) and make R18 larger (try 15k) to get closer, until the base of Q1 and Q2 register within that voltage range.   

[metamatic]

Hi liquid
thanks for your tips anyway. I'll give your buffer biasing scheme a try in the next projects. What improvements would such a scheme provide?

[earthtonesaudio]

By biasing the output near the 1/2 supply voltage, it can accommodate the largest possible signal in both directions.  The caveat is that if you use a single transistor buffer like these, they can source more current from the positive supply (current supplied by a transistor) than they can from the negative supply (current supplied through a resistor), so the 1/2 supply point is only a rough approximation.

Op-amp buffers can generally sink or source current equally well, so they will give a more linear response (and if you use a quad op-amp for the whole circuit, you'll end up using fewer parts as well).

Alternatively, you can sometimes get better performance from the stock 808 schematic by substituting JFETs or MOSFETs in place of the BJTs, with no other changes.  FETs have naturally higher input impedances and can result in lower noise for this circuit.

[liquids]

Yeah...likewise, i'll try it in simple terms, since I'm no engineer!

the 10k/10k resistors act as a voltage divider. The cap to ground from this junction is meant to help 'keep the voltage more stable,' it seems, though I think in real life it doesn't work that well.  But if you're +V is 9v, you should get near 4.5v (1/2 x 9v = 4.5v) at this Vref junction.

R3/R13 do two things - the value (510k) sets the input impedance.  In simple terms as I understand it, it's medium-large value (in comparison to the 10k resistors, for example) is so that there isn't loading on the input signal and the frequency range of the guitar is preserved.  Smaller value resistors will cause loading and loss of signal and high end.

It's a trade off - if you replaced R3/R13 with a 5K or lower, you might actually keep the base around 4.5v, thought the trade off is that would really degrade your signal tonally.   Increasing the resistor to 1M or 510k as it is, your signal is preserved tonally, but since it's so large in comparison to the two 10k resistors forming the voltage divider, the 510k 'can't keep up' and voltage is 'lost' by the time it gets to the other side of the resistor, so one side of the 510k sees 4.5v, the other side sees 2.6v.    Boooo.  Bad (though common) biasing.

If you were working from scratch, you could instead take a 1M (or 820K, ideally) resistor from +V to the base, and another 1M from base to ground.  This would likewise be a voltage divider that would get you in the correct voltage range and about the same input impedance.  but this would be "noisier," in short, than the two 10K resistor junction. It's not uncommon though.

As earthtones said, a op-amp based voltage diver is a much more stable Vref source.  Stable in the sense that, if one was implemented here, you could keep the two 10K resistors feeding the input of an op amp.  No matter how large the value resistor you used for R3/R13--to 'feed' the base of the transistor from the op-amp based output/Vref, you'd get about the same voltage on 'both sides' of the resistor.  I Hope that makes sense.

As a compromise, since it might be overkill and impractical in this and other circuits to add an op amp, you can over-compensate and make the basic voltage divider bias higher than 1/2 voltage--so that the 'losses' are calculated in, and in the end keep the base around 5v, give or take.  For example, replacing R17 with 4.7k and R18 with 15k, say, should get the Vref above 7v, and hence, with the losses, the base would be closer to 4.5v, making a more ideal base-bias for the buffer to "accommodate the largest signal possible in both directions."

In the end, this may make little or no audible difference to you, although it may...it's a whole 'nother thing as to why biasing the base about .7v above 1/2 V+ is a best--earthones explained it well in a few sentences.  But in the meantime, keep it (and op-amp based voltage dividers) in mind for future buffers, etc, if nothing else.