khstudio

2520 - FABIO's - DC Offset !!!
« Reply #20 on: March 19, 2008, 11:14:01 PM »
Quote from: "peter purpose"
Fabio's latest are based on the 76 Melville A5A , reversed by yours truly.
10 transistor CCS.

peter


 :thumb:

Sorry guys,
I didn't want to start any controversy about Fabio's or Peters amps.
...they do sound great. I was just concerned about the offset I measured in the Pultec SS circuit when using them. My lack of knowledge gets me in trouble sometimes.  :oops:

It's nice to know what Model they are & were they came from. :grin:

PETER,
Is the schematic for these TOP SECRET or what
:?:

there's a link to my email below - if you don't mind, I'd like to have it.
I really don't feel like reverse engineering one... if I have problems.
Not to mention I'd just plain like to see it.

Thanks
Kevin ~ KHStudio


2520 - FABIO's - DC Offset !!!
« Reply #21 on: March 20, 2008, 04:24:42 AM »
<<PETER, Is the schematic for these TOP SECRET or what ?>>

Well of course it is.
Fabio doesn't dish out the schematic at my request.

<<if you don't mind, I'd like to have it.
I really don't feel like reverse engineering one>>


Well I really don't feel like sending it out.
I destroyed at least four good 2520s (set in f**kin concrete) and took years to get at the truth in order to draw that schematic.
You have at your disposal an unpotted example.
Give it a go.

peter
http://www.lazpro.com
I reserve the right to not sell to idiots

khstudio

2520 - FABIO's - DC Offset !!!
« Reply #22 on: March 20, 2008, 07:04:15 AM »
Quote from: "peter purpose"
<<PETER, Is the schematic for these TOP SECRET or what ?>>

Well of course it is.
Fabio doesn't dish out the schematic at my request.

<<if you don't mind, I'd like to have it.
I really don't feel like reverse engineering one>>


Well I really don't feel like sending it out.
I destroyed at least four good 2520s (set in f**kin concrete) and took years to get at the truth in order to draw that schematic.
You have at your disposal an unpotted example.
Give it a go.

peter


I did not realize how much went into reversing them. :oops:

I understand your position & I'm thankful that you shared your hard work. :thumb:
Kevin ~ KHStudio

Greg

2520 - FABIO's - DC Offset !!!
« Reply #23 on: March 20, 2008, 08:05:32 AM »
hey Kevin:

Re-read PRRs post. Take a look at your bias resistor... i.e. the one hanging off the non-inverting terminal as a way to reduce your DC offset. I believe PRR feels it is too high for a BJT input decive... like the 2520. Perhaps reducing that value with help (not eliminate) your DC offset.

Also, considering you are using a fixed gain stage, I would lean more towards an inverting configuration (yours in non-inverting) with a DC trim resistor. This might yield better results. Then flip the polarity back with your output transformer.
Greg Stein
New Orleans, LA

nielsk

2520 issues still unsolved
« Reply #24 on: March 20, 2008, 09:22:15 AM »
Peter, would you be so kind as to take a look at this:
http://web.comhem.se/linusandersson/Micksch2520BC_r1.JPG
it is the latest version of changes to the Fabio 2520 board circuit showing the changes made trying to get it to work properly. It still does not (work properly, that is), even with the current source there is a very high frequency osscilation. If you can hint at what to try without giving up the farm, it would be greatly appreciated
we have been posting on this topic here:
http://www.groupdiy.com/index.php?topic=4796&postdays=0&postorder=asc&highlight=2%2A5%2A2%2A0%2A&start=0

khstudio

2520 - FABIO's - DC Offset !!!
« Reply #25 on: March 20, 2008, 04:11:45 PM »
Quote from: "Greg"
hey Kevin:

Re-read PRRs post. Take a look at your bias resistor... i.e. the one hanging off the non-inverting terminal as a way to reduce your DC offset. I believe PRR feels it is too high for a BJT input decive... like the 2520. Perhaps reducing that value with help (not eliminate) your DC offset.

Also, considering you are using a fixed gain stage, I would lean more towards an inverting configuration (yours in non-inverting) with a DC trim resistor. This might yield better results. Then flip the polarity back with your output transformer.


Greg,
Thanks. I was talking to Pat last night about setting up my Make-up amp differently. The schematic IS exactly how Pultec did it on their SS model but that doesn't mean it's the BEST option. Although people like the SS Pultec as is, Pat did mention that when this was designed, the test equipment wasn't as advanced back then & it's possible they just used their ears & liked this setup's sound... who knows.

The bottom line is, I'm not stuck to using it... & thought about trying other things (like your idea) or just set up an API 325 Line Amp, etc...
When I etched the PCB's, I made them "Universal" so I could do anything I want.

Right now... BOTH channels "Match" very well... so changing 1 channel & comparing will not be hard. (this was not the case a week ago. They didn't match)

About the HYBRID:
It still sound good but after further use (during sessions & tracking) I found it seems to "Clip" pretty easily. I didn't seem to have or notice this problem with the TX on the input.
I feel this is because the 600:600 input TX, loaded with the 620r drops the gain (I think, aprox. 6dB ?) but when using the HYBRID, there is NO drop at the input because it's set up for UNITY Gain. This will cause the signal to "Bang" into the 2520 (make-up amp) harder.

So I'm going back to plan "A" with the input TX (Stock Setup)
I've already made a few different 'Buffer" options to try, test & listen to.

After I settle on the input. I will tackle the Make-up amp.
Kevin ~ KHStudio

Greg

2520 - FABIO's - DC Offset !!!
« Reply #26 on: March 20, 2008, 04:50:48 PM »
Quote from: "khstudio"
Now, the SS Pultec isn't set up exactly like the 312 but I wouldn't think it would cause the offset to change. :idea:  :?:

Kevin, in the other thread you stated it was exactly like the SS Pultec, and here you say it is not.

Discrete opamps are not plug and play devices so to speak. Some have BJTs on the front end... some have JFETs. The bias current of these two devices are very different, usually in the nA for BJTs and in the pA JFETs. Among other things, the feedback cap needs to optimized as well.
Greg Stein
New Orleans, LA

khstudio

2520 - FABIO's - DC Offset !!!
« Reply #27 on: March 20, 2008, 05:07:04 PM »
Quote from: "Greg"
Quote from: "khstudio"
Now, the SS Pultec isn't set up exactly like the 312 but I wouldn't think it would cause the offset to change. :idea:  :?:

Kevin, in the other thread you stated it was exactly like the SS Pultec, and here you say it is not.


I NEVER said the SS Pultec was exactly like a 312... I'm not sure what you're reading or mis-reading?

I did say my unit is set up like a SS Pultec, like Pat's schematic.

The only reason I'm referencing the 312 at ALL is because of the DC offset issue.
Kevin ~ KHStudio

Re: 2520 issues still unsolved
« Reply #28 on: March 20, 2008, 05:45:56 PM »
Quote from: "nielsk"
Peter, would you be so kind as to take a look at this:


I'll ave a butchers.
http://www.lazpro.com
I reserve the right to not sell to idiots

 

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