Ampex 600 mic pre compressor build

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industrialarts

Well-known member
Joined
Oct 1, 2020
Messages
130
Hi

Mentioned this in another post, got a working prototype so far. Like most projects, the details are killing me o_O So here is a design from a Nidiot.

First thing, this is supposed to be grungy, not HiFi - I am doing this as a project for a friend and we agreed that it was OK if sounded a little nasty. We didn't want to make just another 600 type preamp, I did that for him already. I opened my big mouth and said "hey, I bet I could make a compressor in there" so it's all my own fault. That said, it sounds pretty good so far and things are working well. I get at least -20db reduction, after that it starts to sound a little crunchy.

I'd love any criticism of the circuit, I am not an experienced designer and I borrowed a lot from the DOC opto compressor (found elsewhere in the forum) and info from Ian / ruffrecords.

To any answer some questions in advance - I tried to keep as much original as I could or close to original so that's why some of the values are what they are. Other values are from experimentation to make it work, and some others are because that's what I had. The opto I am using is the ‎NSL-32SR3‎ by Advanced Photonix (aka Silonex) is has a 25ma current draw, 10ms release and a lower ON resistance than the VTL54. I could not get the Xvive 53 (part I had similar to VTL53) to work but the NSL-32 works well. Using them back to back was from Ian's info and cleaned up a distortion problem I was having with only one.

The meter circuit is a work in progress and seems more complicated than it should be, so the design is just thinking on paper. Suggestions in the area are really welcome.

Other than that, it's really coming together

Thanks in advance for your help

Mark Springer
Vintasonic
 

Attachments

  • COMP_1113.pdf
    97.5 KB · Views: 63
I would be slightly worried about maximum cell voltage of NSL-32SR3. According to the datasheet
VC Maximum Cell Voltage 60 V (Peak AC or DC)
I don't know your B+ or exact operating conditions of the pentode at the input but it most likely can swing more than 60V peak.

How easily can we destroy opto cell with just voltage or do we need power?
 
No resistors in series on the gain cell LEDs! Oh well, you must be rich.
No, I'm not rich. A more helpful response may be something like "you should consider limiting resistors for the gain cell LED"

Like I said, I borrowed from other designs and the DOC doesn't use them, so I didn't consider it. I will take a look at it again, perhaps I should use limiting resistors
 
Hi

Mentioned this in another post, got a working prototype so far. Like most projects, the details are killing me o_O So here is a design from a Nidiot.

First thing, this is supposed to be grungy, not HiFi - I am doing this as a project for a friend and we agreed that it was OK if sounded a little nasty. We didn't want to make just another 600 type preamp, I did that for him already. I opened my big mouth and said "hey, I bet I could make a compressor in there" so it's all my own fault. That said, it sounds pretty good so far and things are working well. I get at least -20db reduction, after that it starts to sound a little crunchy.

I'd love any criticism of the circuit, I am not an experienced designer and I borrowed a lot from the DOC opto compressor (found elsewhere in the forum) and info from Ian / ruffrecords.

To any answer some questions in advance - I tried to keep as much original as I could or close to original so that's why some of the values are what they are. Other values are from experimentation to make it work, and some others are because that's what I had. The opto I am using is the ‎NSL-32SR3‎ by Advanced Photonix (aka Silonex) is has a 25ma current draw, 10ms release and a lower ON resistance than the VTL54. I could not get the Xvive 53 (part I had similar to VTL53) to work but the NSL-32 works well. Using them back to back was from Ian's info and cleaned up a distortion problem I was having with only one.

The meter circuit is a work in progress and seems more complicated than it should be, so the design is just thinking on paper. Suggestions in the area are really welcome.

Other than that, it's really coming together

Thanks in advance for your help

Mark Springer
Vintasonic
It's been over a year since your original post and your follow-up post.....what progress have you made on this project?

Have you finally worked out all of your component values yet?

I have noticed over a long period of time that lots of members on this forum who are interested in designing some type of audio signal-processing equipment use -- "components I had laying around" -- and not necessarily the * CORRECT * values for their circuit design. While I can understand that.....up to a point.....it seems to me that if you have any idea and/or intent of offering your circuit to this forum for others to build and use, then.....I'd spend the time to calculate and determine what the * CORRECT * values need to be using the standard capacitor and resistor values charts!!! > THAT < makes sense to me!!!

[I am not an experienced designer] -- Don't be afraid to ask Ian @ RuffRecords any questions that you may have about either circuit design in general or vacuum-tube circuit design specifically. He will be more than happy to assist you in any way that he can. Tell him "Midnight Arrakis" sent you, OK???

[I am not an experienced designer] -- Once you have determined and have figured out all of the correct component values and you finally have a schematic that is all debugged and verified to be -- how and what -- you want it to be, then you can let me know and I will assist you with creating an electronic schematic, a PCB and the mechanical chassis design. THAT'S what I do. (See the attached PDF files).

/
 

Attachments

  • JBW -- 3-Page OrCAD Electronic Schematic.pdf
    115.5 KB · Views: 1
  • JBW -- VME Rack ASSY DWG.pdf
    85.6 KB · Views: 0
  • JBW-Designed - 1U 12X8 Audio Switcher Rack-Chassis.PDF
    534.9 KB · Views: 0
  • JBW-Designed - 4U-Broadcast Exciter Prototype Rack Chassis - 3-Page.pdf
    788.4 KB · Views: 0
  • JBW-Designed -- SolidWorks mini-Portfolio+FAA Projects+Camera_2108.pdf
    4 MB · Views: 1
  • JBW Designed -- RCA Video Racks + Mechanical & PCB Designs.pdf
    285.1 KB · Views: 1
Thanks for your inquiry. Surprising you would ask now because I finished it up earlier this year but the owner just recently used it. Says he loves it, does what it was supposed to do. I was going to post the finished schemo but I don't have picture of the finished project to go along with it. While I took a some pictures of my work, I failed to take one of the front panel. Both he and I have been busy so I haven't bugged him about the picture but I'll get one soon and post the results.

As far as your advice regarding my not being an experienced designer - thanks, much appreciated. While I have been an audio tech and engineer and systems designer for over 40 yrs I don't consider myself any kind of design engineer - I got my engineering degree (AA not BA) long ago and started in doing tech work soon after, never got into design. Nowadays I mostly just repair and occasionally mod gear; the Ampex was a mod that got a little out of control. I don't much like the math part so my method is to just get close and then fix it from there. That and borrowing from what's already out there usually gets the job done.

One caveat regarding the Ampex: If I were to do it again, I don't think I would approach it that same way. The stereo aspect of it was a little too over-achieving; I think it would be a better mod to make it mono and include a line input, using the second pentode to drive the compression side. I am buried in repairs right now but it did turn out well, so as soon as I can I'll post the results.
 
Thanks for your inquiry. Surprising you would ask now because I finished it up earlier this year but the owner just recently used it. Says he loves it, does what it was supposed to do. I was going to post the finished schemo but I don't have picture of the finished project to go along with it. While I took a some pictures of my work, I failed to take one of the front panel. Both he and I have been busy so I haven't bugged him about the picture but I'll get one soon and post the results.

As far as your advice regarding my not being an experienced designer - thanks, much appreciated. While I have been an audio tech and engineer and systems designer for over 40 yrs I don't consider myself any kind of design engineer - I got my engineering degree (AA not BA) long ago and started in doing tech work soon after, never got into design. Nowadays I mostly just repair and occasionally mod gear; the Ampex was a mod that got a little out of control. I don't much like the math part so my method is to just get close and then fix it from there. That and borrowing from what's already out there usually gets the job done.

One caveat regarding the Ampex: If I were to do it again, I don't think I would approach it that same way. The stereo aspect of it was a little too over-achieving; I think it would be a better mod to make it mono and include a line input, using the second pentode to drive the compression side. I am buried in repairs right now but it did turn out well, so as soon as I can I'll post the results.
Good to hear that things worked out well for both you and the owner!!! Makes you feel good, doesn't it???

I can understand that you're busy and that's life, isn't it??? During the past 3-months, I've been in an auto accident that took me about a month or so to recover from, got food poisoning from a local restaurant that really messed up my guts and as soon as I recovered from that.....I started on a new drug for my diabetes that has been tearing up my guts just as bad as the food poisoning had!!! I'M EXHAUSTED!!!

So.....whenever you do get around to calculating and coming up with the -- CORRECT -- component values for your circuit, you can send me your KiCAD schematic and I could then take a look at what it would take to make a piece of equipment out of it. I am now using the new KiCAD 7.01 software, so I should be able to import your KiCAD 5.1 file into it with relative ease.

> While looking at your posted schematic, I noticed that you have a potentiometer titled "MTR ADJ" that goes across the vacuum tubes filament/heater supply. A) What is its value? B) Why would that be placed across an AC voltage like that? C) I am assuming that this pot is like a BOURNS trimpot? Right??? Then.....there is a second pot located over by the meter itself that is also titled as "MTR ADJ". Is the first "MTR ADJ" pot across the heater lines actually more of a "balance" pot? Just wondering.....

/
 
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