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I'm starting to work on a stereo guitar amp using a pair of LM3886 chips. My 2 x 25V 200VA toroid arrived the other day and it JUST fits inside the hammond chassis I plan to use. So, are there any disadvantages/issues with mounting the transformer inside the chassis? Or should I stick to mounting it to the top of the chassis like you usually see.

Amplifier Discussion / Board member RDV passed away.
September 07, 2010, 01:02:18 PM
Since I know he was active on this forum as well I figured I would post a link to a discussion on the diystompboxes forum about RDV (Ricky Don Vance) . It seems he died back in February. Here's a link to the discussion on the other board:

And here's a link to the announcement:

I was browsing around sparkfun earlier today and came across an interesting stereo chip-amp kit. It seems like it might be good for a small amp (2 x 38W at 4ohm load with a single 18V supply) The fact that it only needs a single power rail might make it interesting for a battery powered amp. The inputs are opamp buffered and it includes everything from the basic components/pcb to a pair of volume trimpots, heat sink, standby switch and clipping indicator LED. If I didn't have so many projects already in limbo I might be tempted to pick one up to play with since it's only $30.

Datasheet for STA540:

If anyone tries this out be sure to post the results!
I ran across this peak limiter circuit and was trying to understand how to control the max output voltage. The author gives guidelines on adjusting the attack and decay times but not the limit voltage.  (circuit in figure 2)

My ability to understand what is going on in this circuit is severely limited so I was hoping someone here could shed some light on how the limit voltage is set. Does it have anything to do with the forward conduction voltage of the diodes? I would like to be able to change the max output voltage to something closer to 1V RMS rather than the 1.65V RMS that it currently has. Of course, I could always take the easy way out and put a voltage divider on the output to get the max voltage down where I want it to be, but that seems like a bit of a hack :)
I saw one of these the other day in my local Sam Ash store and was a bit surprised by the size. They claim it's a 200W SS amp and it's quite small. The enclosure is sealed so it's hard to poke around and figure out what's going on inside. The box is pretty heavy (9lbs) so I'm assuming there's a beefy power transformer contributing most of that weight.. Maybe the heat sink as well (although I didn't notice any vents on the back!?!?!?!)

So, does anyone know anything about these amps? What kind of power amp do they use? (discrete? some chip?) I'm assuming they have some kind of digital reverb... Also, any idea where they could come up with a 200W 6" speaker??

It's an odd beast, that's for sure.. but the reviews seem positive so far. Looks like a good amp design to try and emulate/improve upon.
I've done a search and have seen some conflicting reports on what size transformer is required for one or two LM3886 chipamps. I'm hoping to get a clear answer :)
I want to run a pair of LM3886 chipamps in a stereo configuration. They will each be driving 8ohm loads and the secondary windings of the transformer are 22V each. So, the Transformer is 200VA with two 22V secondaries.. Is this sufficient? The two power amps will share a power supply board (all are from

I've seen some posts that claim 160VA is enough to power two LM3886 amps while others have said that a single LM3886 should have a transformer between 160-200 VA.. what's the real story?? :)

thanks in advance for clearing this up.

I ran across a cheap power transformer on Ted Weber's site today and thought I would share. This would be helpful for anyone looking to build a hybrid amp with a tube preamp and a SS power amp. The price is great too at only $12! Do a search for "flea power" on that page and you'll find it.
Here's a direct link to the spec sheet:

275V CT @ 50mA secondaries and 6.3V @ 1A heater secondaries. Should be enough to run 2-3 preamp tubes from the 12xx7 family.
They even have a nice cheap OT for low wattage amps right below it. Perfect for the type of hybrid solution I'm after where the "preamp" section is really just a low wattage tube amp complete with preamp/power amp and OT running into a dummy load and tapped for further amplification by a high powered SS amp.


Amplifier Discussion / Active Big Muff tone control?
March 30, 2009, 01:28:53 PM
I'm wondering if anyone has had any success building an active version of the BMP tone control. I've read the section in Teemuk's book about active tone controls but I'm still not confident that what I have will work (I have not yet breadboarded it). I've attached a rough schematic of my idea. Do you guys think this will work? There's a DC path from the output back to the inverting input, so the bias offset can make its way back. Adjusting the tone pot shouldn't affect the overall gain of the opamp since there's no DC path to ground in the feedback loop.

Finally... Can anyone point me to a tutorial for simulating these circuits in LT Spice? I know how to produce a simple simulation to measure the gain and clipping characteristics but I'm not sure how to simulate the frequency response curve.

I was reading a TI document about single supply op amp circuits and found something interesting. Everything I've read up to this point indicates a biasing voltage of half the supply voltage should be applied to the non-inverting input to properly bias a single supply circuit.

This document, however, seems to indicate that this bias voltage can be applied to the inverting input instead. As a matter of fact, this seems to be touted as a preferred method.  Here's a link to the document: Pages 5-6 illustrate the type of circuit I'm talking about.

So, is there really anything to be gained by biasing in this way? Any drawbacks?
I've been playing around with some spice simulations of a basic mu amp. In my case I simulated the first gain stage of the blackface preamp found here:

My question relates to controlling the gain of these stages. In my simulation it seems C1 from the linked schematic (the cap from the gate to the source on the upper FET) has a huge effect on the gain of the circuit. Is this, in fact, the easiest way to control the gain? My simulations didn't show any serious change in distortion characteristics by just changing the cap because it doesn't affect the bias point of the stage.

Is it really that simple? I know simulations aren't perfect so I'm wondering if there's some real-world interaction that I'm missing.

I've attached a LTSpice file of the circuit in case anyone wants to play around with it.
Tubes and Hybrids / Hybrid project started.
January 15, 2009, 02:04:50 PM
I just finished the first stage of my hybrid combo amp. My plan was to build a complete low-wattage tube amp, run it through a dummy load (probably a weber MASS speaker motor) and tap the speaker level to drive a passive tone stack followed by a DSP effects processor and finally into my LM3886 power amp. The combo will likely use a single 12" Lil'Texas neo speaker to keep the overall weight down.

I have completed the first phase of my master plan :) The tube amp is done and tested. It's a firefly from
If anyone is curious I have a 2mb sound clip of my friend noodling around on the amp. Keep in mind that the amp was only driving a 5" Jensen MOD speaker so the bottom end is kinda muddy. Strange, who knew a 5" cone didn't do so well in the low range  :P. The guitar is a Gibson ES 335 with stock humbuckers.

I was crunching some numbers to determine what size power transformer I want to buy for my LM3886 so I decided to create a spreadsheet to make life easy. Sine so many people here are interested in the LM3886 (and other chip amps) I decided to share in case anyone finds it useful.

I provide two types of calculations:

The top table allows you to specify how you want the amp to perform (output wattage into what speaker impedance and with what input sensitivity) and calculates what your rail voltages should be and what to set your feedback-to-ground resistor to in order to achieve the desired characteristics.

The bottom table allows you to specify how your amp is configured (rail voltages, speaker impedance, feedback loop resistors, etc...) and it will tell you what your  output power, gain and input impedance are.

For all calculations I also provide gain in Av and dB as well as peak output voltage and peak output current.
Simply type your known/desired values into the top (blue) section of either table and the values will update in the bottom section.

Please see the notes for additional info.

I would appreciate any comments and/or suggested improvements.

I've been looking for a decent (yet simple) speaker/cabinet simulator circuit to integrate into a little preamp project I'm working on. I want to keep the part count low and the board small and am willing to compromise some (ok, quite a bit!) of accuracy to do it. In other words, I want something passable, not perfect. I've come across a couple of designs that seem to fit the bill but have a question.
This one is a direct rip from a Marshall amp with an XLR output. I do have one concern though, only the first two op amp stages have the non-inverting input referenced to VB (bias voltage, presumably). Why is that? Is it because the second two stages are not AC coupled to the first two stages? Does the bias voltage carry through? I would think the voltage would be off because the first two stages have gain. Will the design work as-is? I also believe I should add a DC blocking cap on the output of the last stage, correct?
Here's a link to the full schematic of a Marshall amp that uses this circuit. I'm not finding any answers there because it uses a bipolar supply rather than a bias voltage like the first link I posted. The section of interest is IC3 (b - e) which is on the right side of the first page about half-way down. You can see right above it that the IC is powered by two voltage regulators (78L15 for positive supply and 79L15 for negative).

If anyone is curious, here's another design that is even simpler (using only one dual op amp) I don't have a schematic for it so I'll have to create one from the PCB design. This seems to be an AMZ design but the schematic is no longer on

[edit: Oops... I forgot to include a link to the real marshall schematic :D]

I'm going to be doing some work on an amp for a friend but I wanted to get some info up-front. The amp is an old Marshall SuperLead combo (Solid state)

The speakers are 12" celestion greenbacks with no markings aside from the date code (at least that's what I'm told, I haven't had a chance to look at them personally).  The schematic seems to show two 4Ohm speakers in parallel however 2Ohms seems to be pretty low for an old SS amp like this. If the impedance isn't marked on the speaker is there an easy way to determine what it is? I know I can measure the DC resistance but is there a range the resistance will fall in to based on the impedance? For example, if I get 10Ohms for the DC resistance is that pretty much guaranteed to be an 8Ohm speaker or is it possible for it to be a 4Ohm?

The amp is dated from the late 60s while the speakers are from the early 70's according to the date code.
I ordered a set of Real McTube PCBs from Tonepad and after studying the schematic for the Alembic F2B I noticed that could build either circuit with just a few component changes. The alembic seems to get rave reviews as a bass preamp (and some good reviews for guitar as well). Since the McTube doesn't have a tone control I'll just build a separate board for the tone stack components and tap in to the signal using the pads for the gain control pot.

So, would this preamp be worth while for "regular" electric guitar? I'm not interested in any kinds of effects in the preamp like overdrive or distortion since there are plenty of pedals for that. I just want a nice, warm, clean tone to work with. I'll be running the output through a digital effects board (reverb, rotary speaker, chrous, blah blah...) and adding a master volume control before running the whole shebang into an LM3886 power amp.

So, any thoughts? Is the schematic found here: suficient to recreate the much acclaimed tone of the F2B or is there some other black magic "mojo" involved that I'll be missing?

I am in the process of building a combo amp based on a LM3886 chipamp. The power amp section and power supply are built so now I'm focusing on the preamp section. Based on various posts I've read on here I've settled on the JFET based Fender Blackface clone found here: Now I'm investigating the best way to add reverb to the design. I would like something integrated in to the amp rather than an external pedal/box of some kind. The Stage Center reverb circuit seems like a pretty simple design that gets good reviews so I'm leaning that way. I have read the section on spring reverb in teemu's book but,as usual, I still have a couple of questions.

-Where should the circuit be placed in the signal chain? Obviously the stage center was designed as an effect pedal so it was intended to be placed before the preamp stage. However after studying various amp schematics (which include reverb) it seems the reverb circuit is placed between the preamp and the power amp.

-Assuming the reverb should be placed between the preamp and power amp stages, what modifications (if any) should be made to the standard Stage Center schematic? Is it sufficient to simply build the schematic as-is and wire it into the signal chain? Should the input buffer circuitry be removed?

-What is a good way to control reverb level? It seems the stage center unit has effectively two volume pots feeding into a summing amplifier. One pot attenuates the "dry" signal reaching the summing amp while the other attenuates the output of the recovery stage. Is the first pot necessary?

I'm sure this has been brought up countless times on message boards such as this but a quick search of the history here didn't seem to yield much so I'll go ahead and ask.

with a solid state power amp, how do you decide what impedance speaker to use? I know all the simple electrical stuff that the same power amp will output a lower wattage through a higher impedance speaker (basic ohm's law, same voltage swing through higher impedance = lower wattage) and that you have to make sure not to exceed the maximum current capacity of the power amp... but how about the speakers themselves? Is there any tonal advantage to choosing the 8Ohm version of a particular speaker over the 4Ohm? (or vice versa).  How about loudness? Most manufacturers only give the SPL for one impedance, even if the same speaker is offered in other impedances...

So, here's where the rubber meets the road. I'm trying to choose an affordable(ish) speaker to use with my LM3886 power amp and I'm not sure if I should go with a 4ohm or 8ohm. I plan to keep the rail voltages at a reasonable level so the current draw should be fine regardless of which choice I make.

Obviously, the 4ohm will mean the amp is "putting out more watts" but will that make the 4ohm speaker significantly louder than the 8ohm assuming all else stays the same? How about the tone? I know this is highly subjective and varies from speaker to speaker, but is there a general guideline regarding the sound quality of 4ohm vs 8ohm drivers? One of the speakers I'm considering is the Jensen Mod 12-70 which is available in 4/8/16 ohm configurations.

I know this is probably a vague question but I'm a bit lost. I know any of the choices will "work" with my amp (not going to blow anything up) but what configuration will extract the most from the amp (highest output while still maintaining good tone).

As always, thanks in advance for any and all advice you can give!
Amplifier Discussion / Ruby circuit too clean.
September 28, 2008, 10:36:59 PM
So I built a ruby circuit (slightly modified) to use as a preamp to my TDA2005 power amp and while it has good tone and is quite loud I can't seem to get the amp to "break up" or create any overdriven distortion even with the volume and gain cranked.

Here are the differences between what I built and the ruby schematic:

-removed the filter/zobel network on the output formed by R4 and C7 (not really needed since I'm not driving a speaker directly)
-reduced output cap to 1uF (again, not driving a low impedance load so such a huge cap isn't needed)
-Added a "standard" Big Muff tone control between the preamp (ruby) and power amp.
-Running at 12V
-Added the "grit switch" mod from the noisy cricket

Other than that, it's a standard ruby/cricket circuit. My initial tests were done without a grit switch. Due to the frustratingly clean output (loud as hell, but clean) I decided to add the grit switch and have my buddy try it again... I'm still awaiting results but I'm not too hopeful. I'm using a National LM386-4 and have tried a number of MPF102 fets in case I'm getting too much attenuation from the buffer.. no dice.

Does anyone have any idea what could be keeping the circuit clean? Could it be the 12V giving lots of clean headroom? Or the fact that the amp isn't driving a low impedance load?

I would like to be able to achieve results like this:

I want to run the TDA2005 circuit at 12V minimum to maintain good output to the speaker but may try a voltage divider or regulator to get 9v to run the ruby preamp and see if that helps give some overdrive/clipping sound.  Any other suggestions?

Thanks in advance,
Amplifier Discussion / Proper use of shielded cable
September 08, 2008, 10:37:34 AM
I have a couple of basic questions regarding the use of shielded cable within an amp. Here goes.

1) Which end of the shield should be grounded? I know this is a generic question but I'm struggling with it. I know I should only ground one end of the shield to avoid ground loops, but which end? Lets say I'm running a shielded wire from my preamp board to a volume pot. Should the shield be grounded at the pot or at the board?

2) How many leads for a mono 1/4" audio jack? Since the barrel of the 1/4" plug is ground, can I just connect the shield and use a single conductor inside for the signal (tip)? Or, would it be better to run two-conductor shielded wire from the jack to the preamp board with the ground and signal on shielded conductors with yet another connection to ground for the shield itself?

Thanks for any advice.

I've been asked to see if I can repair an amp for someone. It's a Hudson Electronics "All Solid State Amplifier Model 592". I can't seem to find any information online about this brand of amplifier.

I'm hoping the problem is just in the power supply wiring so a schematic is probably not necessary... But I am curious if anyone knows anything about these amps. Here's what I have been able to observe so far:
-Has three numbered inputs (presumably different gain levels?)
-Controls: Tone, Volume, Reverb, Intensity, Speed
-Reverb tank is mounted to the chassis in the top of the cabinet
-Has an ~8" 10W 8Ohm speaker
-Has two un-labeled 1/4" jacks on the front panel, presumably some sort of preamp output/input like an effects loop(?)

If anyone is curious I can post some pictures.

If anyone knows anything about this amp please share :)