Triad TY-250p Ne5534 saturation/balancing circuit

[Jatchley]

I’ve been playing around with the Triad TY-250p. Stuck a 1.5k+1n zobel on it secondaries and hooked it up to the scope and did some messing around with 7vp-p signal from 20HZ to 20KHZ. It was pretty fine and clean. Shoved 600mV of offset into it and just barely started to get some flattening on one side of the waveform. So we ran a Fender Rhodes retrofit preamp into it as an unbalanced OT of sorts and it sounded nice. A lil bit of mid forwardness and a small Low end bump to my ears. That inspired me to whip up a very cost effective line level saturation circuit schematic using 2 of the ty250p and some NE5534s. I’d love any feedback. I was reading all those posts about biasing opamps into class a and thought why the heck not? Still trying to work out

The resistor from V+ to U1 output.

the emphasis circuit ,

the input R,

and both opamps R to 0V at the non inverting inputs.

Also is the trim ok? The resistor to ground makes me think I could play with that for a deeper attenuation.

Should I put a cap before the primary of the OT?

Also wondering if there’s a better opamp for U2? I’d like a buffer that matches or exceeds the slew rate of U1. I guess what’s the point of having U1 at 13v/uS if the output is at 6V/Us?

My use case I guess is to make 4-8 channels of these to use between an unbalanced mixer insert and an A/D converter or to just use it as an analog insert in my daw for some affordable “analog mojo” or to balance and have some drive for a cheap synth. With the TY-250p being 5$ and ne5534s being .36c from TI it’d be cheap to build.

 

[tk@halmi]

I love these saturation effects! Using the Triad transformers it is also really affordable and buildable from through hole components.

With unity gain at the second opamp I don't think you need to worry much about offset. The input is already DC decoupled. However, that offset will depend on your ? resistor.

I would even use an NE5532. Slew rates will make little difference when you are going for the saturation/compression effect.

The one thing I would never use again is biased opamp output stages. I have played with that a lot in the past and to me it is not worth the hassle. Also, when the conditions are right and the output stage straddles modes it is very unpleasant sounding actually.

 

[Jatchley]

Update: gonna try a 2.1khz emphasis and de emphasis with the values of 7.5k/10nf with 1K to 0V (emphasis) and 6.8k with 100n to 0V (de emphasis).

Also forgot to ask what a good value for the series pot from T1s primary to 0V. 100ohm? 500ohm?

Will the 4PDT switch POP without a cap from U1 output to Emphasis input?

 

[Jatchley]

Update:

 

[Jatchley]

Am I correct in assuming this? Stage 1 has 35db of gain. Both transformers give me a loss of -6db. Both filters give me another loss of -6db. But the differential in the second picture I posted would yield me +6db so this thing has a gain of 29-30db

 

[Jatchley]

Here’s a stepped gain version. Again would love any tips for optimization. I’m kinda new to working with opamps. I wouldn’t be surprised if there’s something I’ve missed or miscalculated. Thanks!
View attachment IMG_0683.jpeg

 

[ToBSn]

Here’s a stepped gain version. Again would love any tips for optimization. I’m kinda new to working with opamps. I wouldn’t be surprised if there’s something I’ve missed or miscalculated. Thanks!
View attachment 145219

Just my 2 cents:
What is this resistor on the first opamp coming from the positive rail and connects to opamp out?

For a active circuit you can try to spilt secondaries of the transformer windings. Use one winding to fead your second opamp stage. The second winding as feadback with pot in the first opamp stage.

 

[merlin]

Since you already have 51R in series with the primary you're gong to want a much bigger pot to ground than 10R if you want to make a useful difference to LF distortion, like 1k ohm at least.
You don't particularly need the .1 caps from the power pins to ground, but they won't hurt.
Is there any point using an NE5534 when you can buy an NE5532 and have a spare opamp available for something?
You're not implementing any DC offset so you may struggle to get true magnetic saturation (separate from LF distortion) if that's what you were intending?

 

[Jatchley]

Since you already have 51R in series with the primary you're gong to want a much bigger pot to ground than 10R if you want to make a useful difference to LF distortion, like 1k ohm at least.
You don't particularly need the .1 caps from the power pins to ground, but they won't hurt.
Is there any point using an NE5534 when you can buy an NE5532 and have a spare opamp available for something?
You're not implementing any DC offset so you may struggle to get true magnetic saturation (separate from LF distortion) if that's what you were intending?

Ok my first thought was higher than 10R like 1K for that series resistance pot. But I second guessed myself. I’ll change it back. I did want to play with adding offset. Where would I inject the offset? Fb network of u1? Non inverting input of U1? Straight into primary? Into Center tap of primary? I wasn’t sure about this.

 

[Jatchley]

Just my 2 cents:
What is this resistor on the first opamp coming from the positive rail and connects to opamp out?

For an active circuit you can try to spilt secondaries of the transformer windings. Use one winding to fead your second opamp stage. The second winding as feadback with pot in the first opamp stage.

If I split the secondaries wouldn’t that change the ratio to 1:1/2? Would I lose half my gain doing that?

 

[Jatchley]

Something like this for offset? I think this would put min of .05vdc and a max of .6vdc

 

[merlin]

There's various ways you could add DC (the DC current is what matters rather than the voltage). I'd probably start with this as a very simple option, but don't forget that it will load U1/the filter, so you might want a switch to have a truly 'off' position?

 

[Jatchley]

Awesome thanks for the help! Now I want to pin down the filters. I’m trying to do something like the riaa curve I guess for switchable pre/post emphasis. I imagine it like a slant gradually boosting above 2khz to 20khz and gradually cutting below 2khz to 20hz and the exact same thing inversed after the transformer to bring it back to flat. Is this something only accomplished with additional active stages? Can it be accomplished passive like I drew it? If so would I need to up the gain on the differential stages? I tried a few different filter calculators and kept getting different results. Not entirely sure about losses. I know that with no filters low frequencies will saturate first but id like the option of low or high frequencies to saturate.