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Messages - willpirkle

Schematics and Layouts / Roland JC-40 Schematic
April 11, 2021, 05:32:10 PM
Anyone got the schematic for the new(er) JC-40 with 2x10" speakers? I am aware of the awesome JC-120 ultimate thread on this forum and also found the JC-50 schematics here as well. But now specifically seeking the JC-40.


Schematics and Layouts / Re: Gallien Krueger 100 MPL
March 08, 2021, 08:06:55 AM
This is the complete service manual, scanned from my paper version that I got from Bob Gallien back in the 90's -- it has all of the gory details including the digital programming. It is almost 80MB in length and won't fit as an upload, so I added a link from my website below. I have no plan to remove the link, but an admin may want to store it in a more permanent manner on an SSGA server for future use.

This is a digitally controlled analog product; e.g. it uses vactrols for channel switching and other routing. It is also a treasure-trove of interesting circuits (log amp for the compressor, use of the LMC835 to implement the GK active EQ filters (in addition to the graphic EQ), noise reduction ckt, even an R/2R DAC).

The schematic pages were repeated within the documents, so there are two sets of them (didn't have time to go back and remove them).



This is a mixed mode amp (voltage and current feedback). Neglecting the current feedback branch (which changes the power delivered to the speaker), the gain of the amp in this configuration is 1 + (R7/(R6 + ZC4)) where ZC4 is the impedance of C4 at a given frequency. At DC, the impedance of C4 is infinite. This causes the (R7/(R6 + ZC4) a term to become 0.0. Then, the DC gain of the amp is unity. As the frequency increases above 0Hz, the impedance of the cap drops, until it is an effective short. Then, the gain of the amp is 1+(R7/R6). Between DC and the short-circuit frequency, the gain rises from unity to 1 +(R7/R6), forming a low frequency shelving filter.

Rod Elliott has a nice discussion of current feedback (aka "constant current amplifier") on that site as well. And, this kind of mixed mode feedback is generally limited to guitar and bass amp applications. And, massive unending arguments among audiophiles.

The C4 cap being electrolytic (and polarized) is non-consequential to the gain equation, and does not limit the voltage swing.

dmeek — now *that* is awesome. I want one!
I only have this one:

It looks like yours would be in the next set, but if the power amps are similar to these (which I think will be the case) then the bias is supplied with the Vbe multipliers Q9/Q10 and Q26/Q27 (first schematic, p7) which aren't adjustable.

Teemuk's book on SS amps (free, links at this site) is a great resource for explaining some of the more esoteric designs that are guitar/bass amp specific. It also includes a few variations on the Vbe multiplier. If you already know the tube theory, then his book will be easy to get into. Highly recommended.

I'm using this TDA1517 stereo amp kit in one of my audio electronics classes. I built and tested it over Christmas and it is stable at 4 ohms. For $12 (Jameco, $27 on Amazon) it might be worth a try for your application. The output power will depend on the supply voltage and current capabilities.

- Will
The Newcomer's Forum / Re: Kustom 50-2 Amplifier /70’s
December 07, 2020, 06:30:00 PM
Have u tried these guys? no Kustom parts per se, but they've got a ton of pots for amps.
Figure 6 shows the external volume pot as 10k-log for line-level sources.
I don't know why I love this, but I do!
I have heard that instructions for the XPR to XPRa mods are out there...somewhere... and that the mods are extensive.

TIA if you can hook me up

- Will
Schematics and Layouts / Re: Scholz UDG Schematic
July 01, 2020, 05:32:41 PM
Yeah, there's one on for $750 (WOW!) but I am interested in seeing the double clipping stages.
Schematics and Layouts / Scholz UDG Schematic
June 29, 2020, 09:02:42 PM
Has anyone traced out the Ultimatum Distortion Generator?

Amplifier Discussion / Re: Complete Rookie
May 07, 2020, 10:37:44 AM
Amplifier Discussion / Re: HH ic 100
May 02, 2020, 05:29:28 PM
Do u have the schematic?