Parametric tone control Question

[90scaraudio]

I've built the DSELF PREAMP v13.01and it works wonderful and now I have a smaller chassis that I'd like to fill. I was looking at this old design, it's actually a 4 band parametric. but I only need 2 bands. I did a bread board test and hooked it up to my Wolverine amp, it actually sounds very good, to my ears.
So before I send the Cad drawing to Fabrication, I was hoping someone here can take a look at the schematic to see if I made any blunders or anything else.
Yes Its old school but I have all the parts.
Thanks all.
Scott

 

[madis64]

If you only need two bands then why do you prefer parametric instead of a traditional shelving bass-treble solution?

 

[justanalogue]

I guess the choice of the centre frequency matters for him, or the Q factor of this design...

 

[90scaraudio]

The chassis has holes in it already and did not want to use a 4 band solution.
So I'm guessing there is nothing wrong with the circuit as is?

 

[90scaraudio]

If you only need two bands then why do you prefer parametric instead of a traditional shelving bass-treble solution?

I've owned a Yamaha C-50 for about 40 years and like how it can add or subtract tones.

 

[90scaraudio]

The thing I'm worried about this design is the input impedance and the balance.
"Rod Elliott - ESP talks about (P-226) the input impedance is determined by R1, which can be up to 100k with a bipolar opamp (e.g. 4558, LM4562, etc.) or 1MΩ if you use a JFET input opamp (e.g. TL072, OPA2134, etc.)."

This has 27k for R1,2. Should these be increased to 1M?
And he also uses a 10k pot for balance in P-02.
Should I change the balance pot to 10k?
I'm not sure.

 

[madis64]

and like how it can add or subtract tones.

Your current schematics simulates roughly to this (RV3A and RV9A having mid values):

A shelving solution would give something like this:

 

[90scaraudio]

Any decent reading you can point me to about shelving circuits?

 

[madis64]

...about shelving circuits?

There is not much to read actually (googling shelving vs peaking should provide enough links though) , take any amplifier's tone control circuit which has two (bass-treble) tone control buttons and start simulating from there.
Center frequencies may vary and the dynamics can be tuned by changing filter component values.

In short - band based equalizers produce "bumps and valleys" in frequency response, shelving tone control produces (usually) continuous flow.
Just for experimenting I once converted a 5 band equalizer to a 5 band set of shelving filters - it worked somehow but in real life turned out to be unuseable.

 

[90scaraudio]

Thanks for looking and taking the time. Much appreciated.
Scott

 

[madis64]

An example of Marantz PM64 treble control simulations (effect of changing one capacitor value) - I did not like the original sound, it was kind of hollow:

 

[90scaraudio]

I did a couple edits to the circuit and ordered the PCB's from OSHPARK.
I'll post a pic when done and how it sounds.
Thanks all.
Scott

 

[90scaraudio]

Final...
I ended using the bass/treble circuit from the Yamaha C-50. The First stage is from P-266 fig 3 and I set it to about 10dB of gain.
The stage after the tone circuit is from P-88 fig 2 and then to Volume/ bal and final stage from p-88 fig 4.
To handle any turn on/off noise, I built a circuit that keeps the Lt/Rt outputs across 100 ohm resistors to ground for about 4 second when unit is turned on.
The Yamaha also used this type of circuit for muting. There is about a +/- 10 dB boost/cut in the range of 100Hz-500Hz and 1kHz-5kHz.
It sounds very good and does not have any noise issues when turning pre-amp on or off. I have it connected to my Wolverine amplifier and it's dead quite to my ears, with the volume all the way down.
Scott