Adventures with the DS-1

[antonis]

No..

[Rob Strand]

Has anyone tried to make a discrete TA7136?

The easy way out is to look at the schematic for the Boss WAZA craft DS1w.   They use a discrete opamp.  It has some specific design features which look sort of like what the TA7136 has.

As far as emulating the TA7136 it's got a lot of parts.   There would be some motivation to simplify it.

I put together a spice model for the TA7136 but the original schematic in the datasheet doesn't have the values so you have to guess the values.   I know someone on one of the audio groups did a similar thing.   The guessed values in that thread are somewhat different to mine.   I at least tried to match the open loop response and equivalent input noise.  I also tried to set the bias current to be in keeping with the use of Widlar current sources being for low current.  I didn't compare the two models since the other model seemed like it had too much bias current on the first stage.

As with many IC designs the current sources can do funny things when the opamp overloads because the current sources share a common reference voltage.  While the Boss WAZA Craft design definitely does not try to copy the internal schematic of the TA7136, it does seem to have specific design features to emulate how the TA7136 clips on the negative rail.  The Boss WAZA Craft design is quite complex so I assumed they needed to add those design features in.  I didn't model the WAZA craft circuit in spice.

I did design a much simplified circuit in spice and tried to broadly match it to my TA7136 spice model.  One design hurdle was to squeeze more gain out of the simplified circuit.   For small signals it seemed to match the TA7136 spice model.   As part of simplifications I removed a lot of current sources.  When I compared the clipped waveforms of the TA7136 spice model and the simplified circuit I could see significant differences.    I played with it a bit but it seemed like some of the current sources need to be put back.    That's going to blow out the idea of having a simplified circuit.

At the time I left it at that.    I didn't build it to see if I could hear the difference between the simplified circuit and the TA7136 or Boss WAZA Craft circuit.

I might have left out some details but that's the general idea.   At the end of the day a solution exists in the Boss WAZA Craft circuit.

[FleshOnGear]

I've been curious about how different op amps can change the sound of the DS-1 circuit. The design I'm working on is different enough that it should be obvious that I'm not trying to copy the sound of any particular iteration of the DS-1. Josh Scott has said that the old DS-1 sounds identical to the newer DS-1A, but I've never played a vintage unit to compare myself. What influence does the TA7136 have on the tone? How is it different from a recent DS-1?

I've been fascinated with the DS-1 circuit, mainly because of the different ways the distortion is produced with this circuit - the combination of a hot BJT boost, distorting an op amp, and clipping diodes at the end. I tried to keep those aspects of the design while taking care of some things I thought were problematic about the sound, mainly the output volume. I did use a socket for the op amp, so I'm looking forward to experimenting with different chips. I guess I'm trying, ultimately, to not lose too much of the essence of the DS-1 sound.

[Elijah-Baley]

If you are interesting in the DS-1 circuit, I'm building a version with a lot of mods. I speak about it here:
https://www.diystompboxes.com/smfforum/index.php?topic=129737.msg1254836#msg1254836


I have to finished that, but the circuit si completed.

[FleshOnGear]

If you are interesting in the DS-1 circuit, I'm building a version with a lot of mods. I speak about it here:
https://www.diystompboxes.com/smfforum/index.php?topic=129737.msg1254836#msg1254836


I have to finished that, but the circuit si completed.

I started reading that thread, but there's a lot there to digest. I have similar goals - basically warmer and more output.

One thing I tried to do was eliminate the pinching distortion from overloading the transistor boost on transients. I did this by DC coupling it to the previous buffer. Unfortunately, I had to end up putting a capacitor there anyway to get the pre-distortion EQ right. The end result is still less harsh sounding than the stock circuit, though.

I also tried to get cute with the tone control. I've actually changed it quite a bit since posting the original schematic I was using, and it's now simpler and warmer sounding. I'll post a revised schematic when I'm done messing with it.

To increase the output I went pretty extreme with the clipping diodes initially. I have since removed one 1N4148 on each direction to reduce the clipping threshold a bit, and there's still plenty of output volume.

Today I spent some time experimenting with different op amps. I only really heard differences with the gain set low, at edge-of-clipping distortion levels. At full gain, as far as I can tell, the differences disappear. My favorites out of the batch I have are the TLE2022, TLC2272, MC1458, and the LMC6482. These chips seem to transition from clipping to clean a bit more gracefully than others. The differences are very subtle, for sure.

I'm getting close to a final version.

[Rob Strand]

I've been curious about how different op amps can change the sound of the DS-1 circuit. The design I'm working on is different enough that it should be obvious that I'm not trying to copy the sound of any particular iteration of the DS-1. Josh Scott has said that the old DS-1 sounds identical to the newer DS-1A, but I've never played a vintage unit to compare myself. What influence does the TA7136 have on the tone? How is it different from a recent DS-1?

Back in the day I'll admit I didn't like the DS-1.

What's interesting is the opamp on the DS-1A is completely different to the DS-1.  It doesn't have the same output stage configuration.  Yet the DS-1w seems to have elements from the original DS-1 opamp.

Some other odds and ends:
- the DS-1A adds the diode to prevent swings below the rail.
- the DS-1w adds a pair of clipping diodes to the input of the opamp.

There are small differences all over the place.

I'm not aware of the TA7136 having input protection diodes.  They aren't shown on the schematic but that doesn't mean they aren't there.   It's an input configuration which might benefit from protection.  I looked at the details of this stuff 15 or 20 year ago.

I suspect someone spent a lot of time on the DS-1A experimenting with different opamps and configurations until that matched the *sound*.