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Old 13th April 2016, 12:29 AM   #1
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Default Home-made tube amp ghost notes?

Hey guys, I'm trying to improve the overdriven tone of my amp. I'm loving the way it sounds for individual notes, but when more than one note is played at a time, you can definitely hear an undertone. I'm not sure whether that's just intermodal harmonics from the notes being played when heavily distorted or from a design problem in the amp. The undertone is almost completely inaudible when playing clean.

I've heard that ghost notes can come from power supplies which are not adequately decoupled. I've attached the power supply in my amp for you to see. Yes, there are a lot of caps. When I built the amp, I didn't know I could get away with less. One would think that should help, in this case.

The least decoupled node is V5, and despite that the schematic says 620R, there's a 560R resistor in there because it's all I had on hand. My math says 1/(2pi*22u*560) = 13Hz, which may not be enough. Hopefully that's all that's causing my problem.

The other nodes are:
V4 - 1/(2pi*22u*1k) = 7.2Hz
V3 - 1/(2pi*22u*1k) = 7.2Hz
V2 - 1/(2pi*22u*1.2k) = 6Hz
V1 - 1/(2pi*22u*4.7k) = 1.5Hz

The amp is also using a star ground, which I'll replace soon with a bus.
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File Type: png Power Supply.png (64.2 KB, 133 views)

Last edited by steeledriver; 13th April 2016 at 12:38 AM.
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Old 13th April 2016, 11:16 AM   #2
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Ghost notes caused by inadequate power supply filtering will show up when you play a single note. They are Intermodulation between the note you play and the power supply ripple.
If you are only hearing ghost notes when you play two or more notes I suspect it is intermodulation associated with the distortion in the amp. You might be able to tell by listening carefully to the pitch of the notes. For example, if you play two notes which are a perfect 5th, the difference frequency produced by intermodulation is an octave below the lower note.
Assuming it is IM distortion, one suggestion is to reduce bass before the distortion stage(s). Many high gain amps have only partial bypassing of the first stage cathode resistor (e.g. only a 0.68uF bypass cap) to achieve this.
Another suggestion is to experiment with the biasing of your distorting stages. I have read (somewhere on the internet) that symmetric distortion (which produces only odd harmonics) does not create IM difference frequencies. (Although I don’t know the theory to support this, as yet.)
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Old 13th April 2016, 11:30 AM   #3
Tesla88 is offline Tesla88  Italy
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I know is a stupid question but, is your guitar in tune ? When you play more than a note you mean a chord ? If strings are not well in tune you get dissonant chords .
Also bear in mind that with overdrive you always get the sum and difference of the notes played. You get "weird notes" when you play notes with a small interval. An exeple is playing 5 fret on B string while bending at 7 fret on G string, i mean your playing an E and bendind a D to E . The less the interval the more you can hear a lower note.
If your guitar is in tune and play a power chord with bridge pick up, it must sounds good.

If chords sound weird the amp design can be the issue, most amplifiers designed for light overdrive sounds muddy when hard drived.


A complete schematic would help.
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Old 13th April 2016, 02:29 PM   #4
jgf is offline jgf  United States
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Not related to your question, but important: The transformer primary should not be connected to safety earth! you've got four connections drawn on the IEC jack, normally there would be three, so it's not clear what's what there. Hot and neutral from the IEC jack should connect to the transformer primary, and earth should connect directly to the chassis.

Last edited by jgf; 13th April 2016 at 02:31 PM. Reason: clarity
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Old 13th April 2016, 03:07 PM   #5
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jgf - You are right, and I drew the schematic wrong. The primary is wired to hot and neutral, and safety earth is to the chassis. I didn't have a symbol in PSpice for the IEC jack, and I definitely drew that one wrong. I'll go fix it in my file.

Tesla88 - I made sure the the guitar is in tune, and the notes are indeed musically related to the interval I'm playing. It must be intermodulation then. And the amp was designed for light overdrive, but I've been making changes to push it harder.

Malcolm - Thanks for the reassuring post. I'll try partial bypassing to see if it helps. I'm experimenting with LED biasing instead of Rk/Ck the amp right now. All of my 12AX7s are biased at 1.7V from the red LEDs I have, which is roughly in the center. The problem existed when I had fully bypassed resistors too, and also when my second and third gain stages were hot biased to try to get asymmetric clipping - just my anecdotal two cents. I'll give this a shot and let you know if it helps!
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Old 13th April 2016, 03:35 PM   #6
DF96 is offline DF96  England
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Distortion causes intermods. No distortion means no intermods. You can have both, or neither.

Symmetric distortion causes odd-order intermods (and odd-order harmonics) - typically experienced as compression. Asymmetric distortion causes even-order intermods (and even-order harmonics) - typically experienced as sum and difference frequencies together with 'richer' sound due to the next octave up.
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Old 13th April 2016, 07:08 PM   #7
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Quote:
Originally Posted by DF96 View Post
...Symmetric distortion causes odd-order intermods (and odd-order harmonics) - typically experienced as compression. Asymmetric distortion causes even-order intermods (and even-order harmonics) - typically experienced as sum and difference frequencies together with 'richer' sound due to the next octave up.
Many thanks for that. Could you clarify a little what odd and even order intermodulation is? Am I right in thinking that (f1+f2) and (f1-f2) are examples of even order, while (f1+f1+f2) and (f1+f2+f2) are examples of odd order?

Can you recommend a good reference that covers this?
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Old 13th April 2016, 10:23 PM   #8
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Quote:
Originally Posted by DF96 View Post
Symmetric distortion causes odd-order intermods (and odd-order harmonics) - typically experienced as compression. Asymmetric distortion causes even-order intermods (and even-order harmonics) - typically experienced as sum and difference frequencies together with 'richer' sound due to the next octave up.
Sorry to quibble, but part of that is not quite right.

Symmetric distortion refers to a transfer function which can be described mathematically as an 'odd' function (or a function having 'odd symmetry') and yes that does produce only odd harmonics.

Asymmetric distortion refers to a transfer function which has neither 'odd' nor 'even' symmetry mathematically - it produces both odd and even harmonics, in general.

A mathematically 'even' function is needed to produce only even harmonics. These are not usually found in the audio chain. An example is full-wave rectification. Notice that after full-wave rectification the fundamental itself has been removed, which is correct since the fundamental is the first harmonic (i.e. an odd harmonic).

An odd function is where f(x) = -f(-x)
and an even function is where f(x) = f(-x)

Last edited by Malcolm Irving; 13th April 2016 at 10:53 PM.
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Old 14th April 2016, 12:05 AM   #9
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Double check the bias and plate voltages in each stage to make sure you didn't make a mistake that would cause too much I.M. distortion. Some is often desirable (for a more natural sounding distortion spectrum shape), but too much can suck.

Another source of ghost notes could be Rf energy coming in from the guitar and/or the AC cord. That energy could beat against other distortion products and produce I.M. difference frequency energy that could fall in the audio band. I always put a .01uF 3kV cap across the primary of the power tranny (and the secondary, instead of across each diode, and across each large filter cap), and a passive Rf filter at the guitar input (10K R in series and a 220p to Gnd. for a -3dB at 36kHZ, assuming the guitar source Z is 10k ohms (it's almost always 6K - 10K unless it's a piezo pickup).

I have found that it is also caused by my ear-brain mechanism when playing too loud. My guitar will sound out of tune, even though I just got done tuning it with a digital tuner, and it sounds perfect at lower levels.
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Old 14th April 2016, 12:37 AM   #10
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There is nothing wrong with a well decoupled power supply.

One of my early valve designs had bad feedback which turned out to come from the power supply feeding back from the next stage.
So I now always have an RC on each power stage of the amplifier.
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