Hi All
I'm just starting up my next tube limiter project - it's based on the 'vacuum tube leveling amplifier' designed by E J Jurich.
The designer himself posted the schematic and supporting information some years back - can't seem to find it here tho'. It surely was archived somewhere. Received very little notice at the time as I recall.
The document is now available for sale from Amazon, so I'll just put up schems of the circuit blocks as I adapt them to my 'vision thing'.
Back then I didn't appreciate the novel twists of the design but I am much better able to understand it all now
.. mostly or 'somewhat'. Anyway - it's pretty groovy.
SO - seeing as I have some nice iron coming courtesy of member 'emrr', I wanted a to do another tube limiter to stretch myself goodly-like.
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It's an all-tube stereo compressor/leveller, intended for radio broadcast duties. It is not, as the designer says, a 'brick wall limiter' but is very effective in its intended role - low (ish) distortion, forgiving in usage and reportedly very nice of a thing to listen to !
I think its an inspired piece - well thought thru, designed, implemented, built and photographed! Now, many years later, I am ready and excited to bring it to life at 'my place'
ps my other finalizer is a finalizer!
Running them into el34 pp tube amps and some funky 70s speakers 8)
( as well as some proper Tannoy 8" full range boxes and ss power amps I use for creetical listneng and what not)
------
Apart from interesting implementation details, the design is notable for it's use of two time-constant dynamic compression characteristic.
One time-constant is fast and drives a vari-gm stage, one time-constant is slow and drives a vactrol opto shunt stage.
Now each of the fast and slow gain cells has individual detectors, and are true stereo audio paths.
One detector, the vari-gm side chain, has a kind of 'true power summing' thingo to prevent skewing of the stereo image where one channel is markedly different from the other.
It's all implemented in the feed-back style (rather than feed-foward).
On top of that, the vari-gm stage uses cathode follower fed topology (CV to grids) rather than and plate loaded output (kind of like the australian Trimax limiting amp, except in feed-back mode and without the 'nasty' delay line in the audio path).
Summarising the topology :
- all tube and quite minimal audio path
- all tube detector paths with power boost
- fully independent Left, Right audio channels in a stereo package
- single ended internal gain staging ie. not using balanced gain cell
-> to be more precise, it usesdiff push-pull style amps but across L-R signals. Not as a classic balanced gain cell config but as unbalanced, symmetrical over L-R. The schematic does it better than my words
- fast tube compressor stage with cathode-fed vari-gm gain control cell, 'power summing of left-right' in the detector
- slow vactrol levelling stage with tube driven vactrol shunt-voltage gain control cell, independent left-right detector
- input amplifier features a passive tonestack style with HF/LF trim (-/-5dB) with gain makeup
My personal twists are :
- transformer input debalancing stage
- transformer balanced output, cap coupled parafeed style (class A operation)
- adapted eq to active bax style. range +/- 10dB, possibly a switched break point option
- different tube types, different vactrol type
- change vari-gm gain cell to cathode following outputs for possible lower distortion
- adaption to the vari-gm time constant network for more flexibility
- additions to the psu for HV regulation and utility rails
- additions and changes to the GR indication circuit and meter
plus
- point-point wiring (like the original) but with low-cost racking
- 'tubes and transformers on top' construction style with knobs 'on top and in front'
- standalone power supply with 2m connecting supply cable
- DOA based balanced parallel 'line out' and a hardware bypass ..... for good measure!
AND of course
- a pair of beautiful 'true-wonder' ballistics enhanced 'old timer' vu meters with chip buffers
RIGHT then - tally ho!
I'm just starting up my next tube limiter project - it's based on the 'vacuum tube leveling amplifier' designed by E J Jurich.
The designer himself posted the schematic and supporting information some years back - can't seem to find it here tho'. It surely was archived somewhere. Received very little notice at the time as I recall.
The document is now available for sale from Amazon, so I'll just put up schems of the circuit blocks as I adapt them to my 'vision thing'.
Back then I didn't appreciate the novel twists of the design but I am much better able to understand it all now
.. mostly or 'somewhat'. Anyway - it's pretty groovy.
SO - seeing as I have some nice iron coming courtesy of member 'emrr', I wanted a to do another tube limiter to stretch myself goodly-like.
------
It's an all-tube stereo compressor/leveller, intended for radio broadcast duties. It is not, as the designer says, a 'brick wall limiter' but is very effective in its intended role - low (ish) distortion, forgiving in usage and reportedly very nice of a thing to listen to !
I think its an inspired piece - well thought thru, designed, implemented, built and photographed! Now, many years later, I am ready and excited to bring it to life at 'my place'
ps my other finalizer is a finalizer!
Running them into el34 pp tube amps and some funky 70s speakers 8)
( as well as some proper Tannoy 8" full range boxes and ss power amps I use for creetical listneng and what not)
------
Apart from interesting implementation details, the design is notable for it's use of two time-constant dynamic compression characteristic.
One time-constant is fast and drives a vari-gm stage, one time-constant is slow and drives a vactrol opto shunt stage.
Now each of the fast and slow gain cells has individual detectors, and are true stereo audio paths.
One detector, the vari-gm side chain, has a kind of 'true power summing' thingo to prevent skewing of the stereo image where one channel is markedly different from the other.
It's all implemented in the feed-back style (rather than feed-foward).
On top of that, the vari-gm stage uses cathode
Summarising the topology :
- all tube and quite minimal audio path
- all tube detector paths with power boost
- fully independent Left, Right audio channels in a stereo package
- single ended internal gain staging ie. not using balanced gain cell
-> to be more precise, it uses
- fast tube compressor stage with cathode-fed vari-gm gain control cell, 'power summing of left-right' in the detector
- slow vactrol levelling stage with tube driven vactrol shunt-voltage gain control cell, independent left-right detector
- input amplifier features a passive tonestack style with HF/LF trim (-/-5dB) with gain makeup
My personal twists are :
- transformer input debalancing stage
- transformer balanced output, cap coupled parafeed style (class A operation)
- adapted eq to active bax style. range +/- 10dB, possibly a switched break point option
- different tube types, different vactrol type
- change vari-gm gain cell to cathode following outputs for possible lower distortion
- adaption to the vari-gm time constant network for more flexibility
- additions to the psu for HV regulation and utility rails
- additions and changes to the GR indication circuit and meter
plus
- point-point wiring (like the original) but with low-cost racking
- 'tubes and transformers on top' construction style with knobs 'on top and in front'
- standalone power supply with 2m connecting supply cable
- DOA based balanced parallel 'line out' and a hardware bypass ..... for good measure!
AND of course
- a pair of beautiful 'true-wonder' ballistics enhanced 'old timer' vu meters with chip buffers
RIGHT then - tally ho!