DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 18 Oct 2018, 16:30

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: 27 Jun 2018, 13:32 
Offline

Joined: 28 May 2018, 10:42
Posts: 6
Hi everybody, my first post so hope I'm doing it right. I've worked up an audio compressor design using dual matched JFETs in a bridged attenuator, with Very Low harmonic distortion (LTspice says -200dB, lol).

Current perplexation is about the gain of the feedback loop. Vout gets up to about 500mVp-p and the control voltage immediately ramps up, and it snaps into regulation. The effect is Hard Limiting beyond a certain level, and nothing below that point.

Instinctively you might want to reduce the loop gain for a broader transition from zero to near-infinite compression. The "side chain" works by Vout feeding a PNP rectifier, which dumps current into a cap, the voltage of which controls the JFET gates. If you scale back the current, the time domain response suffers (hopelessly, beyond a certain point).

I hope this makes sense! Can't really find anything credible on Google. Maybe it's somewhere in some antique schematics with a variable ratio.


Top
 Profile  
 
PostPosted: 27 Jun 2018, 21:47 
Offline
User avatar

Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Check out the manual/schematic for the JBL/Urei/Universal 1176 compressor... particularily "Rev E". It has compensation for the -6dB distortion bump.
(too large to attach here)

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


Top
 Profile  
 
PostPosted: 28 Jun 2018, 17:01 
Offline

Joined: 28 May 2018, 10:42
Posts: 6
Very interesting! I was looking at this schematic, it's Rev. N or something.

http://www.gyraf.dk/gy_pd/1176/1176sch.gif

The compression ratios are set up with different voltage dividers from Vout to the peak-detector amp. I think with my design the transition from zero to infinite compression would still be pretty sharp, just at a different Vout levels. There's another bank of switches that appear to control the DC offset of the peak detector. What does it all mean??? Guess I'll spend the next 80 hours running the simulator.


Last edited by philtronics on 28 Jun 2018, 19:03, edited 1 time in total.

Top
 Profile  
 
PostPosted: 28 Jun 2018, 17:59 
Offline

Joined: 28 May 2018, 10:42
Posts: 6
Coolness! It works perfectly, after I changed to a voltage envelope follower (traditional peak detector) instead of the current source charging a cap, where the JFET gate voltage is set only by the feedback loop.


Top
 Profile  
 
PostPosted: 08 Jul 2018, 10:53 
Offline

Joined: 28 May 2018, 10:42
Posts: 6
Curve time! I can set it up somewhat normally, as in the 1st picture, sort of like the 1176, or for constant max output level as in the 2nd picture. Seems like the 2nd way might be preferable, possibly could delete the Output Level control. Any "thoughts" or recommendations? Thanks!


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.



Top
 Profile  
 
PostPosted: 09 Jul 2018, 00:12 
Offline
User avatar

Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
It depends on your intended purpose. I see useful curves for a variety of uses.

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


Top
 Profile  
 
PostPosted: 14 Aug 2018, 10:35 
Offline

Joined: 28 May 2018, 10:42
Posts: 6
Still chipping away at this, for some reason. Does anybody know how the Gain Reduction panel meter works in this schematic? http://www.gyraf.dk/gy_pd/1176/1176sch.gif

The control voltage connects to a FET on the left - is that to convert voltage to dB?

Not to mention, the output AC voltage appears to be powering the meter.

Please advise!

THanks,
Phil


Top
 Profile  
 
PostPosted: 18 Aug 2018, 10:29 
Offline

Joined: 28 May 2018, 10:42
Posts: 6
My understanding is inching along. That JFET in the meter circuit is nothing fancy, just a buffer and level shifter. Also - the meter can be switched to monitor the output voltage.

In other news... just when you thought that Junction Field Effect Transistors were obsolete - the guys at Linear Systems are coming out with some new FETs for Voltage-Controlled-Resistors! This is on the advise of none other than Bob Dobkin, co-founder and CTO of the former Linear Technology Corporation. The first one will be a dual 2N4391, which is PERFECT for this application.

Sooner or later I'm going to have to actually build this suckah! Will advise...


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: Jago and 6 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy