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Messages - willpirkle

Amplifier Discussion / Re: Fender 112 Deluxe Plus
May 18, 2022, 05:42:59 PM
For the MC1436, pin 8 is a no-connect (N.C.) so you can ignore a connection there.

However, be careful. Often, the supply pins are omitted from the main schematic page(s) because layout techs usually put all the op-amp power supply connections and all of their bypass caps in one area, usually separated from the other pages. This makes the schematic cleaner and easier to read, but can be easily mis-interpreted.

- Will
The input amp is configured as a kind of multiple feedback 2nd order HPF. R6, R7 and R8 set the filter gain.

The C1, C2, R4 R5 network is the HPF. Changing the 510k resistor will alter the LF edge of the HPF (changing any of those 4 components is going to adjust that frequency).

As is, the HPF fc is 60Hz, well below the ~88Hz low E-string on a standard 6-string guitar. This stage has no HF compensation in the feedback path, so the HF breakpoint is super high, well into the MHz range (basically, the op-amp internal compensation is setting the 1st order HF edge frequency, so the op-amp itself is going to set that part of the FR).

This is interesting as I would expect to see the standard 22pF - 100pF cap in the feedback loop to stabilize. If you want to soften the HF response, add a cap in parallel with the FB resistor.

Have you determined if the brittle-ness is an EQ thing or a lack of Nonlinearity in the input stage?


Place a graphic EQ (or multi-band parametric) in the FX a loop and set for Clean channel.

Does adjusting this EQ help the brittleness problem? Is there any setting that makes things tolerable/good?

If not, then EQ or filtering mods probably won't do what you want. I'd look at adding a FET based "warm up" stage to the input preamp. My favorite FET is the J113 and I've made many of the Danyuk-based FET triode sim circuits with them - with the Danyuk equations, the J113's only get about +12dB of gain, and you and resistor-divide that back down to unity if needed. And, only requires a few parts, and can be built easily on a small piece of a proto-board.

If EQing fixes the issue we'll enough, then I'd put the preamp and post-limiter circuits in a SPICE simulator, and see what I could tweak/add to mimic the EQ.

My gut feeling is that the lack of warmth is a non-linear issue, and not EQ. But this way you can test that idea. You could also place very low distortion tube-screamer style boxes in the FX loop, or at the input to the amp (normal stomp box style) to further suss out the issue of EQ vs. nonlinearity.

Hope that helps,


You could replace VR10/11 with log pots if you want to - I'd probably leave them alone.

To get back to the original issue, The input preamp has a max gain of (47/2.2) + 1 = 22.36 = 27dB

If you want to lower the gain of the preamp which is the first op amp after the input, you have to either lower the 47k feedback resistor R4 or raise the 2.2k shunt resistor R5. To experiment, you could replace the 2.2k resistor with say a 10k tweaker-pot and raise the resistance above 2.2k while playing through it till you get what you want.

If still too much gain, change to a 50k tweaker-pot. When you get the gain where you want it, either leave the tweaker pot in place, or measure it and replace with a fixed value.

With 6.8k, you get ~18dB max gain. Note that raising this resistor value also lowers the minimum gain, when the 22k gain pot is at max resistance. So changing that one resistor will alter the overall gain range as well as the max gain.

Agree with Phil - simple attenuator in the FX loop is the least intrusive option.
Easiest thing is to google it, and buy a pre-wired module as there is no single de-facto circuit. Google:

active bass preamp schematic

and see what comes up; prices from $20 to $300 for pre-built and numerous schematics on the Images tab,

Found these low cost options at Amazon:


Alex Lifeson recorded Power Windows with a GK250ML, Dave Murray and Adrian Smith from Iron Maiden used the GK2100SEL in the early 90's, Bob Weir used a McIntosh MC-2300 solid state power amp with a Furman preamp in the early/mid 70s. Wes Montgomery used a Standel Super Custom XV in the 60's.
Start with a schematic. Do you have it?
For the caps, set the DVM on AC volts (preferably AC mV) and read the AC voltage *across* the caps, from one plate to the other with power on. The AC voltage should be 0.0 since caps pass AC. If the AC voltage is higher than a few mV then the cap is leaky and should be replaced. You will need a DVM that can measure relatively low voltages (i.e. not the cheapo ones). The absolute DC voltages on one plate won't tell the whole story. There is not a particular voltage threshold above which the cap is considered bad (thus the vague "a few mV") but if it is in the 100's of mV or in the volt range, then it's definitely bad.

The nice thing about the AC voltage test is that you can set the DVM and then very quickly test every cap on the PCB. My old boss would blindly test every cap (even the little 0.1uF bypass caps on op-amps and especially logic chips) as soon as he opened a unit, and very often had the device fixed within a few minutes since this is such a common failure point. For the big electrolytics in the PS, you can usually hear the 60 or 120Hz noise from the speaker as well.

All great points Phil!

If anyone wants to study this concept further, check out the Peavey super-distortion patent (see Fig 3)

You can legally build stuff from patents for your personal education/edification. If you pull out the distortion circuit, it makes a killer distortion pedal. Add pre and/or post distortion filtering of your own design for a personalized distortion box that no one else has!

Happy holidays, and thanks for an awesome website Joe and for all the great contributions from the power-posters. Here's to a rockin 2022!

Speaking of the RCA app book, does anyone have this one in PDF? Ive been trying to track down some of the late 60's and early 70's RCA books, specifically the audio amplifier versions (not the stock power transistor one). Thanks in advance!
The Newcomer's Forum / Re: Orange Crush 12L (No Sound)
September 22, 2021, 07:01:02 PM
Yep, pin 7 is an output, and should be ~0VDC so that IC is definitely toasted (yes, same pin out as the TL072). You could socket it, if you'd like to experiment with other op-amps, or make changing it simple in the future if it fails again. I couldn't find the schematic for the 12L, but I did find the 20L which includes over-voltage protection diodes on the input. Assuming those are also there on the 12L (2 diodes connected to pin 3), then changing the op-amp should do the trick. But Enzo may have something else to check - he's a super-boss around here!
The Newcomer's Forum / Re: Just joined...
August 06, 2021, 08:11:47 AM
There is a TON of knowledge and a great group of people here - welcome, you will definitely have fun!
Amplifier Discussion / Re: My K-20X
July 21, 2021, 08:08:17 AM
In addition, a line out is designed to drive a high impedance load (>10k, such as the unbalanced input to a mixer, FX return, etc...) while the HP out is designed for very low impedances (8-32 ohms, speakers). A more important difference is that line out on an RCA or 1/4" jack, is designed (by spec) to deliver a specific nominal value of -10dBV, or ~316 mVRMS into that load. There is no formal spec for headphone outputs, so you never know what you're going to get, which is why some headphone outs are really loud and others are quiet, when driving the same load. Headphone outs are typically noisier as they usually use headphone (low power) IC amps, rather than op-amps but in many cases you can't really tell the difference (high gain guitar preamps, etc...).