670 with solid-state side-chain?

[tardishead]

well the only reason I was thinking about a SS sidechain is that I want to build a stereo comp - and the sidechain is very expensive in terms of Sowter replacement transformers, valves, and extras that are needed from the power supply.
is the best idea to go with a Forsell sidechain and add a gainclone/ chipamp to boost the CV for the vari mu valves?
Kubi - you seem to know a fair bit about the Fairchild 660 670 - have you built one yet? or anything close to it

 

[Larrchild]

Assuming that you are going to keep the audio-path topolgy the same, that being a transformer, a push-pull pair(s) and an output transformer...with enough pairs in parallel to acheive sufficient makeup-gain and put the transconductance "dent" in a good place, then the sidechain design is probably going to be one of these variations:

A high impedance rectifier tube driving the grid-control circuit directly. Too Slow. Bad when you have 8 grids to feed.

A solid state sidechain amp that does the time constants at a low level and scales up the DC control voltage. Much Better and Faster

A solid state sidechain amp with high level rectification and time constants. How a 670 does it, but 35 lbs lighter

An all tube clone of the 670 sidechain with its accompanying weight and mass. That's what I'm talkin bout!

A Gainbloak or STK ??? consumer amp chip might be a fine solution, with regard to providing a fast-risetime, low impedance, high voltage source to control the mu-tubes.
It's currently being done in several commercial products in that manner.

I suspect the imperfections in the 670 sidechain amp, give it some added hot-sauce somewhere in the resultant sound. It also has an important waveshaping circuit in the front end. That would have to be emulated in sand-land, I imagine.

Cheers,
Lar'

 

[tardishead]

Kubi - what are you thinking for the SS sidechain? urei? forsell? etc
or do you have your own design?
do you have a design you wouldnt mind to share?

 

[tardishead]

yes i meant add a cv buffer to urei circuit etc. just use the basic rectifier and time constants.
your idea sounds way better. do you have any designs to show

 

[Larrchild]

I'm thinking that in the 407a mic pre I'm adding a peak limiter to, that I will use the 1st 12AX7 stage from the 670 for waveshaping and then couple at hi-z to a big, plastic HV opamp. rectify, and RC it.

Somebody should make a pc board that plugs into the 6AL5 socket of an Altec 436, and turns it into a real sidechain=)

 

[tardishead]

so I presume that everything else in the circuit is the same? This is a kind of solid state emuylation of the control amp circuit in the fairchild? What about the release/attack networks - the same as on the original fairchild?
This would definitely cut costs down and make it easy to experiment with different tube arrays etc.

What about the threshold control?

thanks Kubi - you the man

 

[soundguy]

[quote author="Larrchild"]Somebody should make a pc board that plugs into the 6AL5 socket of an Altec 436, and turns it into a real sidechain=)[/quote]

jesus, if that isnt the most brilliant idea Ive heard in ages. That could be yuor next project Larr...

dave

 

[tardishead]

variable threshold! I presume you put an attenuator on the front of your sidechain circuit - to set the gain of the control amp. Thats pretty much the same as 670 I think.

so now for tubes - I was thinking of going for either 6ES8/ECC189. 6386 are too pricey and too difficult to replace. Alot of people have mentioned 6ES8 as being a close alternative. I presume I can build the signal amp the same as 670 with the 6ES8 (different pin outs obviously) tubes - or do I have to change circuit values etc to fit these tubes?

 

[analag]

The sound of the sidechain amp translate directly into the final sound of the compression. It's overdriven characteristic is important. For this reason I use no feedback in mine, so that it go into clipping a lot gentler like only tubes can. You should hear how this stops and sustain an 808 kick.

analag

 

[tardishead]

Kubi - have you experimented with the 670 signal amp yet?
what tubes do you recommend?

 

[tardishead]

hey Kubi
do you know of any 8:600r transformers available on the market. Old or new?

 

[tardishead]

what are 600 to 8ohm transformers usually used for. I cant find any.
Can 70volt 100 volt line transfomers be used. What is the impedance of 100 volt line systems?

 

[tardishead]

yeh thanks Kubi - youre right I should do some maths instead of just asking questions here.
so I can use a speaker transformer 70v line 10watts. The only problem is like you say frequency response - alot of these transformers are between 100hz and 9khz. Finding a good one might be tricky.
As for the tubes - I think I will go for 6BA6 tubes - they're pretty cheap.
Has anyone devised a good test procedure for matching tubes for the Fairchild 670? I have done searches but have not found much.

 

[CJ]

Just match for current. That will get you close enough to use the balance pot.

Be careful on the 8:600.

A 600 ohm winding is not designed for 300 vac, which you might generate depending on +/- volts.
But most 600:8 should have ample hi-pot.
Saturation might be a consideration.
Need turns and core to do flux math.
Just a note for your awareness files.

 

[Gus]

Why not an autotransformer on a solid state side chain output?

maybe a 70V pa transformer?

 

[vinyvamos]

So I have found two threads on this SS 670 sidechain topic, but decided to post on this one as it seems to have had more technical talk, even though it is an older thread...

People have mentioned here about the sidechain having a certain sonic imprint on the resultant audio output of a 670. I cannot say myself as I have yet to use either an authentic 670 or any other version. I would however like to understand where that imprint might come from, considering that we are literally just full-wave rectifying the output onto the RC network. Is this tube SC amp so overdriven or not much at all?

I am myself in the planning stages of some kind of 670 build, and I really would like to implement an SS sidechain rather than spends loads of money on that original sidechain design. First of all I am trying to understand the DC threshold control and it's effect on the audio in that stage. Is this bringing the 12AX7 to a cut-off point? Where if we listened to it we would hear the audio crackling away in say a decay of a guitar string? Or have I got it totally wrong?

DC threshold aside, and assuming that it can be achieved in a solid-state world, I was also considering how to extract the audio from the vari-mu stage without any trafos. We only need a small signal to then be amplified by a chunky power amp of at least 20 watt rating (6973 PP = approx. 20w). Could we tap off an unbalanced audio signal from somewhere which is then amplified by the SS power amp? Or choose an output trafo with an aux winding to feed the SS SC amp. I do not see any need for output isolation on this SS sidechain, and I also see it making sense to just run a power amp from say -100V wrt to main GND, and this should be able to generate up to -98Vdc...

 

[alexc]

Well, since you are re-viving an old thread - the answers to your questions are 'yes, yes and yes'  most assuredly.

Keep doing your research and the answers you want can be found .. especially here at 'GroupDIY' 

Some things to look into are similar products to that which you mention  and then the 'diy adaptations' of such. 

I think it's fair to say lots of people have made their own builds of 'this kind of thing'.

Good luck with it

 

[alexc]

Also, as a DIY audio builder myself from a 'cool temp region'  ...  nothing beats a 670 style build for warming the room  [temperature wise]  so the side-chain amp can also be 'nice to have'  if you have the 'watts to burn' 

Cheers to our newest 'cold weather region'  contributor!

 

[gyraf]

To repeat myself from ages ago,

the sidechain is by far the simplest and cheapest part of that design.

And it's hardly a 670 if you change the sidechain, is it?

/Jakob E.

 

[vinyvamos]

OK, I thought it might have been interesting to open up the topic again and come to some solid conclusions with schematics etc, but it seems that I have not done enough searching on here and maybe these solid conclusions have been reached in other threads. Or maybe this is just not an interesting topic to discuss for you guys . No worries!

Yes true, it is not really a 670 if it has SS sidechain, but try telling that to all the folk who are shedding $$$$ on the Stam 670 units now with their SS sidechain ...