Tube compressor/expander RDHB 4 7q7

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T

tardishead

Well-known member
Joined
Aug 11, 2004
Messages
625
Location
Sussex, UK
Hi folks
in RDHB4 in the expander section there is a circuit using 7Q7 tubes which has a reasonably fast attack and release time and looks relatively easy to build. Quite simple and elegant. Is this worth building for audio studio use or can someone see any inherent problems?
It says it was included in an amp that at 30 watts and 20db expension had over 5% THD. Thats alot hey or is that more to do with the nature of the power amp????
I was going to add a push pull 6v6 output section and an input transformer.

There is also an expander with 6l7s

RDHB4expander.gif

 
Sadly we don't all have copies - coule you share the circuit to help get the creative juices flowing?

Ta!

z50
 
cool yeh
the tube guys always refer to it on this site - its a bible if you are into tubes. You must get it - its all over the net
let me sort that schematic out.
 
Looks like a lot of fun! Finding a suitable transformer (at a good price) for the control section might be the challenge.
 
Well, 5% THD at max power was considered good performance at the time. You can bet half of it came from the power amp, but you'll be left with the inherent distortion of variable-mu.
In that respect, heptodes are supposed to behave a tad better, although the main benefit is the reduced feedthrough of control voltage.
 
J Langham has some interesting commentary on expanders.  'Round about p.66


http://www.groupdiy.com/index.php?topic=40371.0
 
7Q7 is also 6SA7; 7N7 may be 6SN7 12AU7.  

Diodes may be 1N4148 or similar Silicon jellybeans.

Attack time is at least 250mS.

Expansion makes some sense with very noisy sources, such as 78rpm records, which can not be turned-up without annoying hiss. Turn-up just the loud parts. Expanders vanished with LPs.

In expansion the THD won't be awful. In limiting it will saturate and clip in heavy limiting.

There is an elegance in having the control voltage on a different grid than the signal voltage. (That R-C signal feed won't work well when control is on the signal grid.) But almost nobody else did it this way, and there may be a reason.
 
lassoharp said:
J Langham has some interesting commentary on expanders.  'Round about p.66


http://www.groupdiy.com/index.php?topic=40371.0
Click to expand...

Thanks for that, that is a great read.

I agree with what is been said about expanders, but I am looking for downward expansion (like gate) which is not discussed in J langham's book.

Anyome know a good downward expansion circuit I can DIY? Tube is possible... he he he
JD
 
PRR said:
7Q7 is also 6SA7; 7N7 may be 6SN7 12AU7.  

Diodes may be 1N4148 or similar Silicon jellybeans.

Attack time is at least 250mS.

Expansion makes some sense with very noisy sources, such as 78rpm records, which can not be turned-up without annoying hiss. Turn-up just the loud parts. Expanders vanished with LPs.

In expansion the THD won't be awful. In limiting it will saturate and clip in heavy limiting.

There is an elegance in having the control voltage on a different grid than the signal voltage. (That R-C signal feed won't work well when control is on the signal grid.) But almost nobody else did it this way, and there may be a reason.
Click to expand...

Thanks PRR for you component substitution suggestions.
I am looking for a downward expansion circuit.
I can't work out if this circuit is what I'm looking for. If it is, I won't mind trying it out.

If not, is it possible to alter the circuit (or that of any vary-mu comp) to do downward expansion (GR below threshold).
My limited electronic experience tells me it should be possible... and quite easy to do... it just about altering the tubes bias in the appropreate way.

Any help PRR?
 

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