SS recipe for tube sound

[benito_red]

In the latest years appeared on the market many guitar Solid State amps adopting CURRENT-DRIVE
in power section.

In order to achieve CD, needs to increase the output impedance of a SS amp in about the same amount of
a tube amp (usually from 1 to 3 times the speaker impedance), this will provide the same interaction as between
a tube power section and the connected loudspeaker.
It is possible to find some web articles about CD for SS guitar amps.
They all claim that CD technic is the natural way to get the tipical tone answer of a tube amp.
There are two methods to increase the output impedance of an amp:

1- adding a resistore in series to the loudspeaker.
2- provide the amp with a current feed-back loop.

I built several SS amp based on TDA2030 chip and made many test on them, both adding a serie resistor in some cases,
and implementing current feed-back loop in other cases.
I made many listening tests A/B swiching on/off CD (preamp has mid scooped tone stack
bass/treble knobs and 1Mega Ohm input).
The speaker was C10Q Jensen in open-back cabinet.

Results: in no case the current-feed back improved the sound as claimed, best results were always with CD off.

Did anybody experienced about it? any opinion about?

[Mike Sulzer]

You can affect the frequency response of the SS-amp-speaker combination in this way, and thus make the sound of the the SS change when played through different speakers (having different impedances with frequency) in a similar way to how a a tube amp does, but only when played clean. If you drive the SS hard, it still sounds nothing like a tube amp driven hard.

In my experience making the impedance high with feedback does not affect the sound in a bad way. I do not know why that has happened for you.

In the latest years appeared on the market many guitar Solid State amps adopting CURRENT-DRIVE
in power section.

In order to achieve CD, needs to increase the output impedance of a SS amp in about the same amount of
a tube amp (usually from 1 to 3 times the speaker impedance), this will provide the same interaction as between
a tube power section and the connected loudspeaker.
It is possible to find some web articles about CD for SS guitar amps.
They all claim that CD technic is the natural way to get the tipical tone answer of a tube amp.
There are two methods to increase the output impedance of an amp:

1- adding a resistore in series to the loudspeaker.
2- provide the amp with a current feed-back loop.

I built several SS amp based on TDA2030 chip and made many test on them, both adding a serie resistor in some cases,
and implementing current feed-back loop in other cases.
I made many listening tests A/B swiching on/off CD (preamp has mid scooped tone stack
bass/treble knobs and 1Mega Ohm input).
The speaker was C10Q Jensen in open-back cabinet.

Results: in no case the current-feed back improved the sound as claimed, best results were always with CD off.

Did anybody experienced about it? any opinion about?

[benito_red]

"You can affect the frequency response of the SS-amp-speaker combination in this way, and thus make the sound of the the SS change when played through different speakers (having different impedances with frequency) in a similar way to how a a tube amp does, but only when played clean. If you drive the SS hard, it still sounds nothing like a tube amp driven hard.

In my experience making the impedance high with feedback does not affect the sound in a bad way. I do not know why that has happened for you."

1- I only tested the amp with clean sound because of course the clipping of a power chip (like whatever SS discrete component) can't help to be harsh. In order to get good overdriven sound I use pedal overdrive MOOER Hustle Drive that I find very touch sensitive.

[nickb]

I wonder if the loop was marginally stable leading to ringing ( or a peak in the frequency response of you more of a frequency domain kinda guy). Do you have a scope and signal generator to allow you test just the power amp for this?

[benito_red]

I wonder if the loop was marginally stable leading to ringing ( or a peak in the frequency response of you more of a frequency domain kinda guy). Do you have a scope and signal generator to allow you test just the power amp for this?

yes there could be the possibility of ringing due to marginal stability. I have scope and signal generator, but discovering subtle marginal stability (that should be done on real load (speaker)) can be tricky, so I preferred to repeat tests using a series resistor (to the speaker) in place of the current loop, in this way the possibility of ringing is out of the question, but the conclusion was identical: I always prefered the sound of the amp without CD. Of course that can be a matter of taste, but I submitted the test to a frend of mine that is professional luthier, and he also liked more the sound without CD.
I have my personal opinion and an explanation about CD, that I'm going to share to people who are interested in, but before I would like eventually to get some report from experienced guys

[GainFreak]

An SS amp that will sound like tube amp preferably at bedroom levels? Uhh, no. It's not happening.

[benito_red]

An SS amp that will sound like tube amp preferably at bedroom levels? Uhh, no. It's not happening.

at bed-room level no tube amp will sound like a tube amp. At bed room level you will not get any advantage from tube amp, the best possible solution is to use a clean amp and put in front a good pedal overdrive.

[Mike Sulzer]

At bed room level you will not get any advantage from tube amp, the best possible solution is to use a clean amp and put in front a good pedal overdrive.

No, at bed room levels you still have the tube preamp. It adds warmth when played clean, and if properly designed gives great distortion. I see no advantage in a distortion pedal if you have a good tube preamp. Also you have the sound of the tube power amp played clean. This is not really the same a an SS amp, although they can be very similar.

[J M Fahey]

Results: in no case the current-feed back improved the sound as claimed, best results were always with CD off.

Did anybody experienced about it? any opinion about?

Mmmmmmhhhh, define "improved"

Sound does change, and in a predictable way: bass increases around resonance, so deepest bass reproduced by that particuar speaker, it also sounds looser; treble/bite increases too, and mids to low mids get lowerd; in a nutshell a current driven speaker becomes more agressive and scooped than exact same speaker and cabinet driven by a standard, high damping (voltage source) SS amp.

Of course now we get into the personal taste area: if you already find your guitar/pedal/amp/speaker buzzy or annoying, CD will make it more so; if you found it dull or lifeless now it will have more bite.

At very low volume (bedroom) we hear bass very poorly, plus a light guitar speaker in a small cabinet does not hve lots of it to begin with, so the slight bass increase won´t be noticed much while the biting treble may be annoying.

Two examples of opposite sounds, both very good within their intended demographics:

* classic Yamaha amplifiers (G50/100 - 112/212/115/410): no CD, smooth flat and clean sound, excellent for Jazz players, very popular in late 70`s , or 80`s.
Include Roland JC120 among these.

* the classic Randall RG100, *pure* CD , buzzy and chest thumping.

Most other SS amps have mixed feedback: half voltage, half current , which is the most popuar combination today, it stood the test of time so there must be something to it.

Of course, these are averages or visible trends, but every individual player has his own preferences

PS: a few amps offer a switch to change from pure voltage ("Hi Fi") feedback to mixed type, such as some Marshal Valvestate rack amplifiers and some Polytone, in both cases to switch from a "PA" setting to a "Guitar" one.

[loudthud]

... because of course the clipping of a power chip (like whatever SS discrete component) can't help to be harsh.

If that's the case, you're not doing it right.

Is yours a single or dual rail design?

[benito_red]

No, at bed room levels you still have the tube preamp. It adds warmth when played clean, and if properly designed gives great distortion. I see no advantage in a distortion pedal if you have a good tube preamp. Also you have the sound of the tube power amp played clean. This is not really the same a an SS amp, although they can be very similar.

under the clipping condition and this was one of the test I did, at bedroom average level of 70dB (measured with phonometer, normal conversation is 60dB),
it is true that in blind test it is not easy to say which is tube and which is SS

[benito_red]

@ J M FAHEY

Let me tell the test I did and my related consideration/conclusion.

tube power amps are told to be CurrentDrive amps due to their high output impedance.
In fact it is not true they are really CD because in order to be CD the output impedance should be
at least 10 times the speaker one.
Now let's have a look at the impedance of a real 8 ohm speaker like the Jensen C10Q:

https://www.jensentone.com/speaker/spec/32

you can see that at resonance, the impedance speaker value is 52 ohm
that means that CD should be accomplished with an output impedance amp of at least 500 ohm.
But that is never the case because a tube amp out impedance is always much less that 500 ohm, realistic
values can be 20 or 40 ohms and even less if you apply negative feed-back.
In those conditions the boosting of basses and hights frequencies will be moderate.

But there is another very important factor to consider:
the diagram of speaker impedance Vs frequency, is obtained with real CD and with the speaker in the air.
When we put the speaker in its cabinet the speaker will be charged with the acustic impedance of the
cabinet itself and that will impact the speaker equivalent electrical circuit in such a way that the change
in speaker impedance will be modest.
Final result is, basses and hights boosting due to increased output impedance and variation in speaker/cabinet impedance will be still
perceptible but not at all dramatic.
CONCLUSION:
In my opinion the only valid reason to implement CD in SS amp is to limit the output current (as a consequence of increased
output impedance) in order to avoid the protections intervention of the amp.
Protections could be a problem especially for power chips, where is not possible to set the threshold intervention,
and nowdays every modern SS guitar amp uses a power chip.
So current feed-back is not used to get tube sound or better sound how they claim, but just for relayability potential issues.

[benito_red]

If that's the case, you're not doing it right.

Is yours a single or dual rail design?

At the beginning I made single rail tests, recently a build a new dual rail power section.

what I did not right?

[Mike Sulzer]

I think what people who know what they are talking about claim is that the output impedance of a tube amp, without negative feedback, is several times the nominal speaker impedance, and that this affects the frequency response in the way that JM said. That is, you have the bass resonance and the gradual rise at high frequencies resulting from the coil inductance.

When you answer what he wrote by saying that tube amps are considered current drivers, you are setting up a straw man and knocking that down rather than dealing directly with what he wrote. They do not have to be pure CD in order for the stated frequency effects to happen.

Thus your conclusion is wrong. If you really do not hear the effect that is described here, there is something wrong with your listening tests, and that casts doubt on what you wrote back in your first post in this discussion.

@ J M FAHEY

Let me tell the test I did and my related consideration/conclusion.

.......

CONCLUSION:
In my opinion the only valid reason to implement CD in SS amp is to limit the output current (as a consequence of increased
output impedance) in order to avoid the protections intervention of the amp.

[benito_red]

I think what people who know what they are talking about claim is that the output impedance of a tube amp, without negative feedback, is several times the nominal speaker impedance, and that this affects the frequency response in the way that JM said. That is, you have the bass resonance and the gradual rise at high frequencies resulting from the coil inductance.

When you answer what he wrote by saying that tube amps are considered current drivers, you are setting up a straw man and knocking that down rather than dealing directly with what he wrote. They do not have to be pure CD in order for the stated frequency effects to happen.

Thus your conclusion is wrong. If you really do not hear the effect that is described here, there is something wrong with your listening tests, and that casts doubt on what you wrote back in your first post in this discussion.

you misunderstood what I told. I agree that the basses and hights boosting effect due to increased out impedance is present and, I also stated that it is perceptible, but not in a dramatic way due to the fact that we will never be in a real CD situation and also that the speaker placed in the cabinet present to the amp an impedence with much less variation
that's all