modding a hybrid amp for a cleaner sound

[bluesrockplayer77]

Hello all,

I have a 15 watt Johnson hybrid amp which I would like to mod for an overall cleaner sound. It has plenty of clean headroom so there's no issue there. Any suggestions on modding the circuit, such as changing cap values, different preamp tube (The current one is a JJ 12AX7), etc. I got the amp for $30 but it sells for $200 new, so I would think even a somewhat expensive mod, if there are any, would be worth the money put in to it.

I've already decided to replace the speaker, I'm just not sure what speaker to get. I've been doing some research and it seems like a Weber speaker would be a good choice, but I haven't heard one yet.

Any suggestions or ideas? Thanks in advance!

[soundguruman]

You can increase the value of the cathode resistors in the preamp, decrease the values of the cathode bypass capacitors in the preamp.
This will make the stage less sensitive and yield less distortion.
But at some point when decreasing the value of the cathode bypass cap, you will also decrease the frequency response, so you can only push it so far in a hi fi application.
Decreasing the value of the plate resistor in the preamp stage will decrease the gain of that stage, and clean up the input sensitivity.
Example: 100K plate resistor change to 47K.
A preamp stage that was designed for TAPE input is too sensitive for a CD player, and distorts. Tweaking as above can clean that puppy up significantly.
Same rule for guitar amps, actually.

[Chuck H]

If the amp has "plenty of clean headroom" I don't really understand the need to make it "overall cleaner". Do you want it to be louder? Is there some aesthetic to the volume control setting that disturbs you? Do you want the amp to be more dynamic?

If you just want it to be clean to a higher volume knob setting you can change the taper of the volume control.

If you want the amp to actually be cleaner then the suggestion to increase the input stage cathode bias resistor is a good one. This will make that stage (the first and most sensitive) a little more dynamic and a little lower in output. If your first stage is a transistor then a simple voltage divider or adjustment to an existing one will do it.

If you want to make the amp louder I would suggest a very efficient speaker. Since you're thinking of replacing the speaker anyhow this is a great way to gain some volume. If you choose a speaker just 3db more efficient than the one you have now it's the same volume increase as doubling your wattage.

[soundguruman]

Actually it takes quite a bit more than double the wattage to achieve a doubling of actual output volume.
In fact it takes 8-10 times the wattage power to double the volume. This is why there is almost no difference in volume between a 50 watt and a 100 watt amplifier, nearly undetectable to the human ear.
This is why there is almost no difference in volume between the 60 watt and 100 watt setting of a guitar amplifier, like a boogie.

Most people do not realize this, but that's how it works out.

So you see, using a more efficient speaker in the correct cabinet, has more effect than increasing the power by double, it's a lot louder than that. And you will hear a huge difference between a cheap speaker and a good speaker. Once you use the good speaker you will never go back to a cheap one, at least not by choice.

[Chuck H]

Actually it takes quite a bit more than double the wattage to achieve a doubling of actual output volume.
In fact it takes 8-10 times the wattage power to double the volume.

This sounds like a correction... (and actually it takes a db increase of about six to percieve twice the volume, so that would be about four times the wattage. not eight to ten times). What I said was:

If you choose a speaker just 3db more efficient than the one you have now it's the same volume increase as doubling your wattage.

And this is accurate.

I worded it that way so that the OP would understand the significance of speaker effeciency.

It's true enough that the stock speaker in that amp may be of low efficiency and a good sounding speaker with a db increase of five or six DB is possible. That would be a very notable increase in volume.

Most of us here have already been down this path and know the significance of speaker efficiency. And hw some speakers, even with a good rep, just don't sound right with some amps. Many of us (me included) have speakers laying around in boxes, barely used, that should be listed for sale (if we'd only get around to it) )

[Enzo]

A doubling of SPL would take 10 times power. Perceived loudness is all relative to the listner and starting level.


6db is the difference between the common high and low jacks of most Fenders.

[Steve Conner]

The trouble with these hybrid amps is, they all are different, there isn't a standard topology like with tube amps. So the advice given here may make no sense in the context of your amp.

For $200 I'd expect to see a couple of 4558 op-amps (where are the cathode bias resistors on those? ) a power amp chip like the TDA2050(?) and the tube stage somewhere in the middle, running with a very low voltage, giving an exaggeratedly tubey sound. Easiest way to clean it up would be to bypass the tube circuit.

A new speaker is always a safe bet, though.

[soundguruman]

A doubling of SPL would take 10 times power. Perceived loudness is all relative to the listner and starting level.


6db is the difference between the common high and low jacks of most Fenders.

It works out to between 8-10 times, depending on speaker efficiency. Thanks for recognizing this little known fact.
Actually we speak of loudness using a sine wave. Distorted, square wave is a bit more efficient in loudness, compared to a sine wave.

[Chuck H]

(and actually it takes a db increase of about six to percieve twice the volume, so that would be about four times the wattage.

OK. I missed that one. Another case of taking things out of context. Not all db ratios are the same. 6db is a 2x voltage increase. A 2x volume increase is indeed 10db. My bad. But it does seem confusing. This from site :
Loudness volume sound level change factor of perceived loudness formula calculate power level noise levels volume logarithm dependence three four fold loudness sound - by what factor does level decrease dependence comparison decibel levels 3 dBSPL 6

Ratio doubling means:
A sound pressure level +6 dB SPL,
a power level of +3 dB
an electric voltage level of +6 dB
a loudness level +10 dB
10 dB more SPL means 10 times increase in amplifier gain (amplification)

So +6db is double sound pressure but double loudness is +10db??? I hope I'm not the only one that gets confused about this sometimes.

[Steve Conner]

SPL is physics. Loudness is human perception. That's where the confusion comes in, Felcher Mungster curves and the like
Sone - Wikipedia, the free encyclopedia

[Chuck H]

Steve, I think it's only a matter of time before you start calling them Herman Munster curves.

[bluesrockplayer77]

I guess what I was trying to say was that I'm looking for a cleaner signal. Exactly, I need more dynamics from the amp.