Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Interesting Soundcraft 1600 mod results
Interesting Soundcraft 1600 mod results
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st February 2020, 12:57 PM   #101
dreamth is offline dreamth  Romania
diyAudio Member
 
Join Date: Nov 2010
Quote:
Originally Posted by sgrossklass View Post
. And unless I missed something, the divider ratio of 16k/3k9 in the ground compensation does not match 100k/22k too well either... why didn't they go with e.g. 10k/2k2 instead?


He said OPA213x had become rather too expensive for his tastes. Also, with those 100ps I reckon a unity-gain stable part would be good, so OPA2227 instead of 2228.
I've seen this in many mixer applications where they don't respect that ratio as to compensate for input impedance.Here's a j-fet input capacitance too.usually they make thousands of tests before choosing some values . OPA2228 is stable for gains of 5x (10k/22k...)and at 10x it will drive 1...1.5nf without compensation...but that 22pf over the feedback resistor looks exactly like a compensation capacitor
Attached Images
File Type: png opa2228comp.png (81.4 KB, 66 views)

Last edited by dreamth; 21st February 2020 at 01:04 PM.
  Reply With Quote
Old 21st February 2020, 03:26 PM   #102
dotneck335 is offline dotneck335  United States
diyAudio Member
 
Join Date: Dec 2012
Quote:
Originally Posted by sgrossklass View Post
Basically looks good. You missed the 3k3 input resistor (R46 or R45?)
I'm not following you here-----?? I don't see a 3.3K Ω R46/45 anywhere.
Quote:
Originally Posted by sgrossklass View Post
.... Q2/4 are likely to be a waste of some perfectly good ZTX450s
So, a BC549 or a 2N3906 here?
Quote:
Originally Posted by sgrossklass View Post
... there is no reason not to leave R7/9 at 4k7 (or is that supposed to be the combination of 3k9+470 || 4k7?)
That's EXACTLY what it is---I figured to just leave the input pad & switch as is.
Quote:
Originally Posted by sgrossklass View Post
..... and you could go down to 1k-1k5 for R3/10 to get the common-mode swing down at this point, but other than that...
I'm not understanding "common-mode swing down"
Quote:
Originally Posted by sgrossklass View Post
...He said OPA213x had become rather too expensive for his tastes. Also, with those 100ps I reckon a unity-gain stable part would be good, so OPA2227 instead of 2228.
Those parts (OPA2227/28) are even MORE expensive than the OPA2134!! Even with my concocted SOIC-8 to DIP-8 adaptors (~74˘), the OPA1642 is less $$$, and a fine performer.
  Reply With Quote
Old 21st February 2020, 11:25 PM   #103
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Quote:
Originally Posted by dotneck335 View Post
I'm not following you here-----?? I don't see a 3.3K Ω R46/45 anywhere.
Interesting Soundcraft 1600 mod results-soundcraft200-inr46-png
Quote:
Originally Posted by dotneck335 View Post
So, a BC549 or a 2N3906 here?
2N3904*
Or most everything else comparable you might have on hand, except maybe a 2N5210.
According to sim, using 2N4403 as inputs yields 1.188 nV/√(Hz) at 2 kHz with 2N3904, 1.212nV/√(Hz) with BC550 (On Semi) and 1.228 nV/√(Hz) with 2N5210, using Bob Cordell's models. (RB = 30.1, RB = 167 - vs. RB = 900 for 2N5210, maybe some Central Semiconductor cheapies - similarly high Rbb' has also been noted for cheap BC550s.) So apparently you still slightly benefit from lowish Rbb' while beta only needs to be moderately high. 2N4401 (RB = 13) also works well, at 1.204 nV/√(Hz). Guess you shouldn't be using parts from the very cheapest manufacturer...
Quote:
Originally Posted by dotneck335 View Post
I'm not understanding "common-mode swing down"
According to simulation, some of the input is being converted into common-mode voltage at the opamp inputs - I've seen 2 Vpp or so. The amplitude of that scales with collector resistor value. So best choose that no larger than necessary to get DC levels comfortably away from the supplies. Thankfully the OPA164x input stage seems to be pretty much upside down from the TL07x, meaning it'll gladly work down to V- and even a bit below.
But don't go too low altogether, as (10k||100p)/Rc determines your opamp noise gain and hence how much remains of your opamp GBW. At 500 ohms the whole thing would be as fast as the original TL072 and configuration, so I wouldn't go below that for sure.
Like I said, 1k sounds like a good compromise.
Attached Images
File Type: png soundcraft200-inr46.png (38.7 KB, 136 views)

Last edited by sgrossklass; 21st February 2020 at 11:43 PM.
  Reply With Quote
Old 22nd February 2020, 12:08 AM   #104
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
PS: Input transistor current varies slightly between various npns, which may explain part of the difference in noise - from 932 ľA (BC550) to 1.03 mA (2N3904). 2N5210 949 ľA, 2N4401 955 ľA. So the 3904 "cheats" a bit as well, and the BC550 actually does decently. Absolutely speaking, the differences do remain minimal though.
  Reply With Quote
Old 22nd February 2020, 12:26 AM   #105
dotneck335 is offline dotneck335  United States
diyAudio Member
 
Join Date: Dec 2012
Yes, I see R46 (3.3K) now; my bad. I guess I just overlooked it because I didn't see any need for it---what does it accomplish?
Yes, 2N3904, not 6---got my polarities mixed up.
Horowitz-Hill shows 2N3904 as rbb=110, not the 30.1 that you mention---????
Actually, I was just gonna leave the NE5532s and circuitry as is. Just get rid of the compromise-filled TL072s everywhere. OPA 1642s on the input boards and LM4562s on the output tape returns.
So, if I may ask your indulgence, what noise/distortion does simulation show for ZTX550/450s with OPA1642?

Last edited by dotneck335; 22nd February 2020 at 12:30 AM.
  Reply With Quote
Old 22nd February 2020, 05:14 AM   #106
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
> what does it accomplish?

Connect a long mike cable but do not connect a microphone.

The stock 94k(?) input will catch all the buzz and half the RF in the room.

A 3k input picks up much less crap.

"Most" console marketers favor the later behavior. Open inputs are not normal. But they happen. And LOUD open inputs un-nerve the potential buyer.

There's a sub-dB loss of S/N but few can tell. Indeed YOU can chose to omit the 3k, and close your ears when you mess with open cables.
  Reply With Quote
Old 22nd February 2020, 10:46 PM   #107
dotneck335 is offline dotneck335  United States
diyAudio Member
 
Join Date: Dec 2012
Quote:
Originally Posted by PRR View Post
A 3k input picks up much less crap.
AHA! Thank you, PRR, for your astute reply!
  Reply With Quote
Old 22nd February 2020, 10:54 PM   #108
dotneck335 is offline dotneck335  United States
diyAudio Member
 
Join Date: Dec 2012
So, the Complementary Feedback Pair works so well in my mic preamp application, I was wondering if it could also improve the performance of this tape head reproduction amplifier by simply replacing Q1 with my Zetex pair (550/450) of transistors and 680Ω resistor. I'd also change the LM318 for an NE5534A. I'm showing a 2N3906 (what was available on SiMetrix) but the actual transistor is a "CD701" which I have not been able to find ANY information on.
Attached Files
File Type: pdf ATR100ReproAmp.pdf (46.9 KB, 8 views)

Last edited by dotneck335; 22nd February 2020 at 11:06 PM.
  Reply With Quote
Old 23rd February 2020, 01:52 AM   #109
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
I do not believe your "Repro head" connection is correct. Here is how I see Ampex's plan:
Attached Images
File Type: gif ATR100-pre-42.gif (13.8 KB, 53 views)
  Reply With Quote
Old 23rd February 2020, 04:41 AM   #110
dotneck335 is offline dotneck335  United States
diyAudio Member
 
Join Date: Dec 2012
Quote:
Originally Posted by PRR View Post
I do not believe your "Repro head" connection is correct. Here is how I see Ampex's plan:
I see your point. Schematic corrected. Also finally found the mysterious Ampex transistor---it's a 2N3964, a low-noise PNP with exceptionally high Beta, which is quite expensive these days.
Attached Files
File Type: pdf ATR100ReproAmp2.pdf (30.8 KB, 11 views)
  Reply With Quote

Reply


Interesting Soundcraft 1600 mod resultsHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Which Soundcraft 200 is this? curtis73 PA Systems 6 8th July 2019 11:06 PM
Soundcraft gb aux help! Anton65 PA Systems 15 1st December 2014 04:38 PM
Interesting results from dust cover upgrade thoriated Analogue Source 1 25th August 2009 09:50 PM
Interesting! ALPS blue velvet pot test results.... ransom peek Chip Amps 7 28th October 2006 04:48 AM


New To Site? Need Help?

All times are GMT. The time now is 09:51 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki