Channel/sum amp
« on: September 13, 2017, 01:22:47 PM »
I just tried the first stage of the SSL 4000 channel line amp on the breadboard, as I want to try it out as a potential summing amp.  I sent input to (55) 57 and grounded (57) 55.  Is this the right approach for virtual earth summing with this amp?

It sounds pretty good, but I'm not really doing full listening tests right now.  I'd say it was a positive result however.  I'm going to compare it to the discrete line amp I have built and then to Ian's Twin Line Amp.

I might have a switch to choose between summing amps.. that's the thought at least.

The SSL 4000 Channel Amp Link:
http://www.ka-electronics.com/images/SSL/ssl_82E01.pdf

Adam
« Last Edit: September 13, 2017, 01:31:42 PM by 80hinhiding »


Re: Channel/sum amp
« Reply #1 on: September 13, 2017, 02:02:32 PM »
Just found the group amps as well here:
http://schematic.danrudin.com/index.php?dir=SSL/SSL_4000E_Service_Manual_Schematics/Section_1__Input_and_Output_Module/

I have some questions because I want to learn more about all this.

What's the inductor doing in the mix amp?  Do you guys ever use bobbin/ferrite core with nice results?  They're compact, and cheap.

The back to back electrolytics there on input and output (positive poles in facing each other) are configured like that to make sure the dc conditions stay the same and the positive supply does not get into the AC?  I was going to configure a mute switch similarly, but hadn't pictured the poles facing that way.

Anyone know what value the bus feed resistors are?  I see a 22K log pot there in the mix amp so I was wondering what the gain is.

Also, the symbol for 100K there in the feedback loop, I haven't seen that before.  What does it represent?

Adam

JohnRoberts

Re: Channel/sum amp
« Reply #2 on: September 13, 2017, 02:23:41 PM »
Just found the group amps as well here:
http://schematic.danrudin.com/index.php?dir=SSL/SSL_4000E_Service_Manual_Schematics/Section_1__Input_and_Output_Module/

I have some questions because I want to learn more about all this.

What's the inductor doing in the mix amp?  Do you guys ever use bobbin/ferrite core with nice results?  They're compact, and cheap.
The inductor in series with the bus is to decouple bus capacitance and prevent instability.

Unless you use a long bus with lots of capacitance you can probably get by with a modest feedback cap (like the 100pF in that schematic).
Quote


The back to back electrolytics there on input and output (positive poles in facing each other) are configured like that to make sure the dc conditions stay the same and the positive supply does not get into the AC?  I was going to configure a mute switch similarly, but hadn't pictured the poles facing that way.

Anyone know what value the bus feed resistors are?  I see a 22K log pot there in the mix amp so I was wondering what the gain is.

Also, the symbol for 100K there in the feedback loop, I haven't seen that before.  What does it represent?

Adam
I am guessing they like 30k resistors... I opened several schematics but didn't see any bus feed resistors before giving up.  Now I need to delete those files.

The 100k looks like a potentiometer.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Re: Channel/sum amp
« Reply #3 on: September 13, 2017, 03:21:38 PM »
Thanks for taking the time John.  I appreciate it.

Re: Channel/sum amp
« Reply #4 on: September 13, 2017, 03:48:47 PM »
I just tried the first stage of the SSL 4000 channel line amp on the breadboard, as I want to try it out as a potential summing amp.  I sent input to (55) 57 and grounded (57) 55.  Is this the right approach for virtual earth summing with this amp?

It sounds pretty good, but I'm not really doing full listening tests right now.  I'd say it was a positive result however.  I'm going to compare it to the discrete line amp I have built and then to Ian's Twin Line Amp.

I might have a switch to choose between summing amps.. that's the thought at least.

The SSL 4000 Channel Amp Link:
http://www.ka-electronics.com/images/SSL/ssl_82E01.pdf

Just found the group amps as well here:
http://schematic.danrudin.com/index.php?dir=SSL/SSL_4000E_Service_Manual_Schematics/Section_1__Input_and_Output_Module/

I have some questions because I want to learn more about all this.

What's the inductor doing in the mix amp?  Do you guys ever use bobbin/ferrite core with nice results?  They're compact, and cheap.

Adam

There's nothing "magical" about that line input circuit (SSL 82E01)
It's just a standard inverting unity gain op-amp based balanced line receiver circuit.
The sound is as good as it's gonna get.

As for a summing amp, one NE5534A (also in inverting unity gain) with 10K 0.1% (I've tried as low as 6K8, it works too) mix resistors is good and low noise enough to sum 8 channels of single ended audio signal :)

Re: Channel/sum amp
« Reply #5 on: September 13, 2017, 04:35:51 PM »
There's nothing "magical" about that line input circuit (SSL 82E01)
It's just a standard inverting unity gain op-amp based balanced line receiver circuit.
The sound is as good as it's gonna get.
Thanks.  And when you feed an unbalanced line to that circuit you connect the + input to ground?  Regarding magic - not looking for a single bullet, just looking for options and to learn. :)  I'm pleased with a line amp design I have been testing based off a document from the early 70s.

Quote
As for a summing amp, one NE5534A (also in inverting unity gain) with 10K 0.1% (I've tried as low as 6K8, it works too) mix resistors is good and low noise enough to sum 8 channels of single ended audio signal :)

Thanks for the resistor value.

Looking at the schematic I thought opamp labeled T2 was handling the virtual earth mix duties (giving gain/trim options) while the opamp labeled T8 was to invert the inverted to preserve polarity?  I could see that opamp T8 was at unity.

Cheers

Adam

abbey road d enfer

Re: Channel/sum amp New
« Reply #6 on: September 13, 2017, 05:03:11 PM »
Anyone know what value the bus feed resistors are?  I see a 22K log pot there in the mix amp so I was wondering what the gain is.
3dB loss in the panpot and 15k bus resistors is consistent with 22k feedback for unity gain.

Quote
Also, the symbol for 100K there in the feedback loop, I haven't seen that before.  What does it represent?
It's the european symbol for trimpots.
« Last Edit: September 14, 2017, 12:13:50 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

Re: Channel/sum amp
« Reply #7 on: September 13, 2017, 05:12:48 PM »
3dB loss in the panpot and 15k bus resistors is consistent with 22k feedback for unity gain.
 It's the european symbol for trimpots.
Awesome thanks!

Re: Channel/sum amp
« Reply #8 on: September 13, 2017, 06:27:24 PM »
And when you feed an unbalanced line to that circuit you connect the + input to ground? 

Looking at the schematic I thought opamp labeled T2 was handling the virtual earth mix duties (giving gain/trim options) while the opamp labeled T8 was to invert the inverted to preserve polarity?  I could see that opamp T8 was at unity.

Cheers

Adam

yea, non inverting input to ground.
add another inverting unity gain op-amp at the output to correct signal polarity.
so basically you either use two single op-amps, or a dual op-amp if PCB space is a concern.

when i mentioned about my summing amp 10K mix resistors, i wasn't referring to SSL schematic, sorry.
and it was without a pan or a mute circuit either.