Converting SS eq. to Tube

[mashaffer]

I have an ADC sound shaper II 12 band stereo equalizer that has some bad switches. I intended to pull the switches out and just hard wire it to operate with eq all the time and just switch it in and out with the tape monitor switch on the amp but I got to thinking (very dangerous)...

I wonder if I could use the chassis and the filter network to build a tube based eq and crossover unit. It could even double as a line stage as well.

Is there any reason why I couldn't use the RCL network of this unit as the basis for a tube version? I assume that the filter network is in a feedback loop but I haven't quite figured out the details of how the existing circuit works (I have the schematic). It looks like the input goes through an emitter follower into some sort of differential stage.

The network consists of a series of 50k pots (sliders) with a series RCL network between the wiper and ground. The two ends of the pots are connected between the bases of the (apparently) differential stage.

So could the network be connected across the output of a CF and used as the input to other tube stages for volume control and crossover? Would some other approach be indicated here? Am I barking in entirely the wrong forest?

mike

[mashaffer]

Here is the Schem. for the EQ. Any thoughts at all on this?

mike

[ErikdeBest]

Hi Mashaffer

I don't know and I didn't look very well at your schematic - but i think that kust putting valves instead of the SS devices won't work, because most of the times SS works with resistors at least a decade below tubes, DC ratings of capacitors are way lower in SS designs, etc.

I am attaching a schematic of a tubed equalizer, so you can compare both.
http://www.jogis-roehrenbude.de/

Erik

[mashaffer]

Danke, aber ich can nicht gut Deutsch. Can you give me a link to the schematic or directions on how to find my way to it.

mike

[ErikdeBest]

Ha, Hi mashaffer

Sorry for not posting the complete link - strange actually. But here you go
http://www.jogis-roehrenbude.de/Vers.../Equalizer.htm

Und noch immer Deutsch, aber die abbildung sprecht für sich!

Erik

[wildswan]

Hey Mashaffer, I'm designing some passive tube eq stuff ( LCR ) at the moment.

The problem is that tubes won't be able to drive very low impedances like this an they're own. On max cut <500Ohms and unity gain ~25K.

You would be better off building a nice discrete opamp with lots of headroom and have the output devices run at a high current.

With faders on a minimum setting, it seems that the minimum impedance of these stages would be very low <500 Ohms (although the signal will be -20dB ) 25k is also quite low for tubes, for passing +4dBu signals.

IMHO a nice discrete solid state opamp with good current and voltage capability is the way forward.

[mashaffer]

Thanks guys. I sort of expected impedence problems but thought I would throw it against the wall and see what stuck. As far as I can tell all of the active circuitry works fine on this unit and it just has bad switches.

I will probably proceed with my initial plan of hard wiring it and using it in my bedroom system. I will save my tube ideas for a bigger project.

mike

[GEirin]

Hi Mashaffer.

You have a service manual of the ADC SS-2 equalizer?

GEirin

[mashaffer]

No, I only have the owner's manual which has the schematic in the back.

mike

[GEirin]

Hi Mashaffer,

Thank for you reply.

You can post to forum a better image (photograph or scanner) witch total diagran circuit? Please.

Thanks

GEirin