Low Power Electric Guitar Amps

[Steve A.]

real small!



BluGuitar - Home of the AMP1

now used by Jennifer Batten and Uli Jon Roth

I didn't think that the OP was looking for a 100W amp... BTW Thomas Blug worked for H&K for many years and designed their Red Box DI's.

Steve

[Steve A.]

Do Celestion Blue or Greenback affect much?
Because 4W ones don't have these speakers, I assume.

There are several different AC4 models. The AC4TV and AC4TVmini have just vol, tone and attenuator controls, with an 8" or 6.5" speaker. The AC4C has a Top Boost circuit with gain, mv, treble and bass controls requiring a second preamp tube, and come with 10" or 12" Celestions (but not Greenbacks.) For $319 or $349 they would sound closer to the AC15C1 Top Boost channel but without the reverb or tremolo. I thought that the AC4C-12 sounded pretty good but on clearance the AC15C1 was only like $100 more.

Steve A.

EDIT I was just checking the schematics and the AC4C preamp is very similar to the Top Boost channel of the AC15C1 and the Marshall-style MV should work better than the PPI cross-line MV on the AC15C1 at low settings.

I also learned that the AC15C1 uses several opamps in the audio sigjal chain... but very intelligently, like to sum up the outputs of the two channels and for the reverb driver and recovery circuits.

The AC4TV model has a topology similar to a 5F2A tweed Princeton with an attenuator circuit (see following drawing.) Perhaps someone can confirm that the two capacitors are electrolytics with the positive terminals connected together, and venture guesses as to wattage and voltage ratings.

[rjb]

Perpetual Noobie Wanders into the Room....

Regarding the attenuator network, are these guesses correct?:

> The circuit is a high-pass filter in the 1W and 1/4W positions, and favors high freqs most in the 1/4W position.
> This is to overcome speaker cone inertia (speaker is less sensitive to higher frequencies when driven at lower voltages).
Or something like that?

Also:
I know that speaker frequency response curves are generally shown for 1W input.
Are there plots that compare a speaker's response curves at different input levels?

[rjb]

Regarding the attenuator network, are these guesses correct?:

Hey, is everyone stumped- or just rolling their eyes and snickering?
Juan? Enzo? Somebody?

[Dave H]

Regarding the attenuator network, are these guesses correct?:

> The circuit is a high-pass filter in the 1W and 1/4W positions, and favors high freqs most in the 1/4W position.

I'd call it a shelving filter. A high pass filter attenuates the lows and passes the highs unchanged. This circuit boosts the highs and then levels off at some point. And yes there is more high frequency boost in the 1/4W position.

[rjb]

I'd call it a shelving filter. A high pass filter attenuates the lows and passes the highs unchanged. This circuit boosts the highs and then levels off at some point. And yes there is more high frequency boost in the 1/4W position.

Thanks, Dave.
And the reason for using a shelving filter rather than a simple resistive divider is...?

[Steve A.]

Thanks, Dave.
And the reason for using a shelving filter rather than a simple resistive divider is...?

Normally attenuator circuits use capacitors and inductors as well to shape the sound so it sounds more natural. BTW I read that this attenuator sounds halfway decent in the 1 watt setting (the 1/4 watt is supposed to be pretty bad.)

I figured that it might be cool to wire up the 1 watt mode to some of the 5 watt amps out there.

Steve Ahola

[rjb]

Normally attenuator circuits use capacitors and inductors as well to shape the sound so it sounds more natural.

Maybe I'm over-analyzing.
This circuit has no tone-shaping in the 5W position, but boosts highs in the 1W position, and boosts them even more in the 1/4W position.
That seems to be doing the opposite of what the Fletcher-Munson curves would suggest https://en.wikipedia.org/wiki/Equal-loudness_contour
So I thought maybe the circuit compensates for some physical characteristic of the speaker....

BTW I read that this attenuator sounds halfway decent in the 1 watt setting (the 1/4 watt is supposed to be pretty bad.)

Aha, that's telling(?)
Maybe at the 1W setting it's a bit quieter with some treble boost, and at the 1/4W setting it's a tinny POS.

I figured that it might be cool to wire up the 1 watt mode to some of the 5 watt amps out there.

I have a Fender Champion 30 DSP that I'd like to use to play electric guitar with unamplified acoustic instruments (e.g. fiddle, accordion).
It's way too loud for that purpose, and I thought I might try a similar attenuator circuit.

[Dave H]

Maybe I'm over-analyzing.
This circuit has no tone-shaping in the 5W position, but boosts highs in the 1W position, and boosts them even more in the 1/4W position.
That seems to be doing the opposite of what the Fletcher-Munson curves would suggest https://en.wikipedia.org/wiki/Equal-loudness_contour
So I thought maybe the circuit compensates for some physical characteristic of the speaker....

I could be wrong but I think it's to compensate for the way the amplifier drives the speaker at higher frequencies. The speaker is inductive and its impedance rises with frequency. The AC4 output stage has no negative feedback and a high output impedance which tends to drive the rising impedance of the speaker harder at high frequencies. When a resistive attenuator is added the resistors reduce the impedance driving the speaker so it isn't driven as hard at high frequency making it sound dull. The capacitors are to compensate for this.

[Enzo]

Opposite of Fletcher-Munson?

Your ears are reasonably flat at loud levels, but the lower the level, the more your ears lop off the highs and lows. I don't know that it is the reason for the circuit, but it seems to me that the more attenuation, the lower the signal level at the speaker, and thus the more end frequency boosts you'd need to sound "normal".

Like a brightness cap on a volume control: the more you turn the volume down, the greater the (relative) brightness effect.

[rjb]

Opposite of Fletcher-Munson?

Your ears are reasonably flat at loud levels, but the lower the level, the more your ears lop off the highs and lows. I don't know that it is the reason for the circuit, but it seems to me that the more attenuation, the lower the signal level at the speaker, and thus the more end frequency boosts you'd need to sound "normal".

Right. That's what I meant. If you look at the equal-loudness contours and consider that the amp probably isn't putting out anything over 5-6 KHz to begin with, it seems that as you lower the signal level at the speaker the more you'd need to boost the lower frequencies to sound normal. Instead, the circuit boosts the higher frequencies.

Like a brightness cap on a volume control: the more you turn the volume down, the greater the (relative) brightness effect.

I was thinking that's what this circuit does.
But Dave H's explanation sounds reasonable to me.
I'll just have to try it & see how it sounds with my amp.

Thanks, everybody.

[eschertron]

I see the attenuator as a 'first approximation' attempt to compensate for loudness. Probably the HF is more noticeable with a speaker of any given size, and so the compensation may be more predictable. That may be enough to satisfy many guitarists. An RC network for emphasizing lows shouldn't be too hard, and you get the added bonus of 'throwing away' all that energy in the mids and highs which helps reduce loudness. My guess is that the LF emphasis network would need more tweaking from speaker size to speaker size though.