Recommend a discrete guitar preamp to install in front of a class-D power amp?

[Timpanogos Slim]

A few weeks ago i bought a distressed Kustom KM-100 keyboard amp at a thrift store for five bucks.

It's popping fuses and i presume that the TDA7293 chip amp is toast. "100w mosfet" was technically true i guess.

At the moment my plan is to remove everything from the amp chassis except for the power trafo and install a 200w or so class-D module and some preamp boards. The Celestion Truvox 1520 speaker works fine and though the original piezo tweeter is a joke, parts express has a variety of decent horn drivers for the treble. The chassis seems plenty sturdy, just needs new handles. Cheap from apex jr.

I'm mainly learning bass these days, and the Kreuzer mostly-jfet bass preamp looks like a fine selection for the preamp section. But even if i install two of those, there's a ton of realestate in the original amp chassis left over.

In a gig or practice situation maybe someone wants to plug a guitar in.

I could spend some time in kicad putting together a layout for the preamp section from any decent opamp-based guitar amp, but is there a layout out there already? Google seems to be stymied by all the links to onboard or pedal centric preamp designs. Haven't found something yet appropriate for wall power, +/-18v supply.

Any suggestions?

[Enzo]

For guitar you don't really need a tweeter anyway.

The original board already has a preamp on it, all laid out and stuffed, the controls all match up with the panel legend. Power supply is already there. Just a thought...

[Timpanogos Slim]

For guitar you don't really need a tweeter anyway.

The original board already has a preamp on it, all laid out and stuffed, the controls all match up with the panel legend. Power supply is already there. Just a thought...

Depends on your definition of "tweeter". A lot of PA horns are crossed over at 1khz. Even a bass guitar has harmonics as high as 4khz.

The original piezo horn might work but it was always terrible, according to reviews of this amp by keyboardists.

Celestion claims this woofer is good to 4khz but i have my doubts about the clarity above 1.2khz or so. Admittedly i haven't measured it - just going by long experience with LF drivers and the fact that's where their published response curve starts to get messy.

Anyway, I suppose the original preamp section might be acceptable. using a single TDA7293 was pretty weak of them though. Particularly with a speaker capable of so much more.

[Enzo]

Other than "acoustic amps", which are basically small PA systems, how many guitar amps can you think of with a tweeter? Bass cabs sometimes have them because the speakers are designed for low end. Try it without one. You can always add it if you must. PAs are not guitar amps, different goals.

Yes, I have never liked th sound of piezos, they are a compromise at best.

[R.G.]

Celestion claims this woofer is good to 4khz but i have my doubts about the clarity above 1.2khz or so. Admittedly i haven't measured it - just going by long experience with LF drivers and the fact that's where their published response curve starts to get messy.

The roll off into messiness is generally taken to account for the speaker's "voice", much of what makes speakers different sounding.

Guitar speakers are not PA or Hifi speakers. The makers don't try to get them to be terrifically smooth responders in the upper mid and treble. The messiness is viewed much like the "beauty spots" that were once all the rage in the makeup of beautiful - or wannabe -women long ago.

I suspect the claim of the woofer being good to 4kHz amounts to picking a frequency where the peaks and valleys of response can no longer be responsibly advertised as a response. It's probably not intended to imply "flat to 4kHz", only that there's significant output up to there, kinda.

Guitar speakers are properly thought of as the acoustic part of an electro-acoustic musical instrument, the electro part being the guitar and amp. Most guitarists don't want a really extended high frequecy response, and complain if they get it. Depending on your subjective criteria, it may be "better' by some measurements, but it's sure not expected, and not what the player thought they'd be getting.

[J M Fahey]

The Truvox is an *excellent* PA/Keyboard/Bass speaker , not so much for guitar.
Frequency response is very flat from very low frequencies to 4kHz as rated and then drops like a brick.

Wide enough for guitar but it lacks the typical strong 10 dB peak between 2000 and 3500 Hz all guitar speakers have, so it will sound "dull" by comparison.
Jazz guitar players will love it, of course.

Considering that your "200W" Class D amp will still put out 100W , since it will be fed from the original PSU,you canīt pull 2 liters of Coke out of a 1 liter bottle, I would first try to repair the original one, which might mean not much more than replacing TDA7293 and a couple extra parts.

Keyboard preamp is excellent for Bass, you might want to boost gain a little or build a relatively simple "Bass preamp in a box" , such as Aguilar and others offer or simply a flat gain pedal, to rise passive bass output to Keyboard levels (gain from 2X to 4X or so).
If you have an Active Bass, you wonīt even need that.

I bet that amplifier already includes a good compressor/limiter, which is essential in SS bass rigs (qnd on keyboards of course); any Ebay you can get will not.

You might build a couple Bass and Guitar premp pedals and carry them in your gig bag, use whateverīs needed, consider this amp a dedicated mini PA.

In fact lots of Guitar players are doing that: they use a complex pedalboard or even Guitar Rig or similar in a notebook, and drive a flat loud "guitar monitor" ... an incredibly versatile setup.

[Timpanogos Slim]

Well yeah. In some sense, a guitar speaker or guitar cabinet is an expensive tone control. There's a blog out there somewhre, don't have the link handy, where someone graphed the response curves of several speaker / amp / cabinet emulation circuits and dsp presets. It's pretty interesting. For bass, most of them have not much under 80hz, a small bump around 100hz, a small dip around 200-400hz, and a gentle rise to either 3khz or 5khz before falling off a cliff.

So yeah, that truvox is a great bass speaker. For $5 it was a real steal at Savers, even if it hadn't come with the cabinet and amp.



I did replace the TDA chip today. Works fine now. For Kustom levels of fine, I guess. The most striking thing about this amp is that when i saw it, at first, second, and third glance i assumed it was designed in about 1978 and sold through maybe 1992. Nope - it's from 2000. Anyway, so far I only tested it with a sacrificial (terrible) 6x9 3-way car speaker. Still need to re-mount it in the cab. Which needs new vinyl cladding, new handles, and new grille fabric.

I've only recently dabbled in amplified performance but I've always had the vague impression that Kustom is a barrel scraper brand. The KMA-100 has not dissuaded me from that impression.

A class-D amp should produce more clean SPL before getting nasty. The TDA7293 datasheet advertises 100W at 10% THD with +/-40v rails. The elbow in the power vs. thd graph for 40v rails is at just over 80w. So, for people who enjoy music, this is an 80w amp. I did measure the power rails at +/-43.5v.

The power vs. thd curve for the IRS2092 reference design, with +/-50v rails and an 8 ohm load, doesn't hit it's elbow until well after 400w.

So with 43v rails and nominal 8 ohm load, granted it would be up to the power supply capacitors to handle transients like in any design, but i should be able to get more clean power output than with the TDA chip.

Anyway - the plan so far was to engineer a flat response in the speakers, which would generally favor the bass and any keyboards or mics that might get plugged in, and if i felt like adding a guitar circuit, it would of course have an eq circuit wedged between it and the power stage.

This would mostly be for screwing around at home with friends, and annoying the neighbors. There might be people who want to hear us, but i question their sanity and taste and i'm reasonably certain i don't want to meet them.

I'm also building what should be a totally decent theremin, which would ideally be paired with something that has reasonably flat response throughout the human hearing range.

[Timpanogos Slim]

There is a ca3080 opamp on the board, but no compressor control on the panel. *shrug*. Google hasn't turned up any KMA-65 or KMA-100 schematics.

[J M Fahey]

So yeah, that truvox is a great bass speaker.
I did replace the TDA chip today. Works fine now. For Kustom levels of fine, I guess.

Then we are talking the same level of fine as Marshall/Fender/Ampeg/Peavey/Crate/Laney/Hartke , etc.

I've only recently dabbled in amplified performance but I've always had the vague impression that Kustom is a barrel scraper brand. The KMA-100 has not dissuaded me from that impression.

You said it twice: "impression". In fact same quality level as Marshall/Fender/Ampeg/Peavey/Crate/Laney/Hartke , etc.
Which I already said by the way.

A class-D amp should produce more clean SPL before getting nasty.

Not with the same supply.
You might get extra 1V peak, not too sure about that, and it might even work the other way.
Both the Class D one and TDA7293 use power Mosfets with similar turn on and saturation requirements.

The TDA7293 datasheet advertises 100W at 10% THD with +/-40v rails. The elbow in the power vs. thd graph for 40v rails is at just over 80w. So, for people who enjoy music, this is an 80w amp.

True, I also consider it an 80W amp.
It is not that "its distortion is 10%" , but to "reach" 100W they allow for some clipping which is silly. Cheap marketing.
If not clipping, distortion is very low.

I did measure the power rails at +/-43.5v.

Thanks. Thatīs unloaded voltage.
That voltage will drop at least 15% , maybe even 20% under full load (also consider much increased ripple at full load) no matter what kind of amp is pulling that power.
So both a linear and a Class D amp will put out about the same peak voltage on the load (speaker), within a Volt or so, if fed from the same supply .... which is what you will do.
There may be a minor power out difference, not significative, definitely not audible.

The power vs. thd curve for the IRS2092 reference design, with +/-50v rails and an 8 ohm load, doesn't hit it's elbow until well after 400w.

You are comparing pears to oranges, Iīm certain they consider the 50V *regulated* and *rippleless* which is possible if using a Lab supply or at worst a tightly regulated SMPS.
With the same conventional supply, both "hit their elbow" at about same peak voltage, so about same power.
And in any case:

doesn't hit it's elbow until well after 400w

is physically impossible even using regulated rippleless +/-50V supplies.
Peak voltage, under best conditions, might reach 48V so 34V RMS so 144W into 8 ohms.
Even if losses were generously low 2V and into 4 ohms, that woud mean 288W RMS.
400W? NO WAY !!!!!!!!
I think that IRS must be a barrel scraper if they cheat so much with specs

So with 43v rails and nominal 8 ohm load, granted it would be up to the power supply capacitors to handle transients like in any design, but i should be able to get more clean power output than with the TDA chip.

Sorry but no.

Anyway - the plan so far was to engineer a flat response in the speakers, which would generally favor the bass and any keyboards or mics that might get plugged in, and if i felt like adding a guitar circuit, it would of course have an eq circuit wedged between it and the power stage.

This would mostly be for screwing around at home with friends, and annoying the neighbors. There might be people who want to hear us, but i question their sanity and taste and i'm reasonably certain i don't want to meet them.

I'm also building what should be a totally decent theremin, which would ideally be paired with something that has reasonably flat response throughout the human hearing range.

Cool , please keep us updated

There is a ca3080 opamp on the board, but no compressor control on the panel. *shrug*.

You wonīt find a front panel control for it, for the very good reason itīs not "an effect" but "a necessity" so Musician should have no means to turn it off or even change its settings.
Such compressor/limiters are hardwired for a reason.

As a side note, Kustom designer Rick Kukulies is so well known and respected by the Industry, that NAAMM itself decided that he had to be part of their Music History Series.
They interviewed him to explain Kustom history and evolution, sadly also explaining how it was (commercially) forced to go from a full American manufacturing company to having stuff built in Asia ... like all others, including staunch American Peavey.
Not their decision, but US Govtīs who changed the rules of the game, so either outsource or disappear.

https://www.namm.org/library/oral-history/rick-kukulies

And after he retired, Kustom kept the high quality design by hiring none other than James Brown, Peavey former Design Engineer ... howīs that ?

James ...who? ....

Ok, ok, read along:

About James Brown

Yes, among other stuff he designed the original Peavey 5150 for Eddie van Halen, howīs that?
Of course, Musicians only care about the long haired, bursting with "attitude" Guitar Hero of the day; none knows or cares about the real hero who brought them that fine piece of equipment to begin with.

End customers know the finished product (of course) and the "famous Artist" who uses it .... few if any at all know who is *really* behind them.

So you might rethink your idea of Kustom being the bottom of the barrel .... itīs actually quite the opposite.

Of course, they do not have the zillion Dollars others have available .... but thatīs not the main point.

Or is it?

[Timpanogos Slim]

So with regard to switch mode amps you're a non-believer. Noted.

My personal experience has been that their advances in efficiency are pretty effective. Clarity and ease on the ears in "real watts" vs. class AB, I think there's something compelling there, for me.

[g1]

I'm not sure what you mean about 'real watts' but if power specs are being published, there should be a standard.
If something gives 'ease on the ears' relative to something else, I would think it would have less power.
Peak power is one thing, but peak or burst is not really what I think people mean when talking about 'real power', and I think that was what Juan was implying with regard to specs.
Like I like to say, if there were amps that could deliver more 'real power' than they use from the wall, we could all generate 60hz and sell it back to the utility companies. (* 50Hz where applicable )

[Timpanogos Slim]

I'm not sure what you mean about 'real watts' but if power specs are being published, there should be a standard.
If something gives 'ease on the ears' relative to something else, I would think it would have less power.
Peak power is one thing, but peak or burst is not really what I think people mean when talking about 'real power', and I think that was what Juan was implying with regard to specs.
Like I like to say, if there were amps that could deliver more 'real power' than they use from the wall, we could all generate 60hz and sell it back to the utility companies. (* 50Hz where applicable )

Would be helpful if they gave the length of time it can output a given power level at a given thd+n with some power supply specs. I agree.

Thankfully most people don't listen to continuous sine waves so peak output power does at least mean something. It's not completely meaningless.

I'm unsure of the VA size of the transformer in this amp. It only has the house markings, iirc. I can look again. I guess i can guesstimate it's size comparing it to other R-cores.

It's basically sort of true that you can get a class a-b to make as loud of a noise as a class d at the same power supply and input signal levels, but my experience is that the class d amp topologies are usually less distorted for a given power level, or at least less audibly distorted. And with far less of the power supply wasted as heat.

[Gnobuddy]

And in any case: is physically impossible even using regulated rippleless +/-50V supplies.
Peak voltage, under best conditions, might reach 48V so 34V RMS so 144W into 8 ohms.
Even if losses were generously low 2V and into 4 ohms, that woud mean 288W RMS.
400W? NO WAY !!!!!!!!

For a traditional output stage, I agree entirely with your analysis.

However, while I'm not familiar with this particular class D module, I've seen data sheets for many current class-D chips that actually consist of two amps operating in bridge mode - the output is an H-bridge, with the speaker fully floating, and the two ends driven by anti-phase AC voltages.

This arrangement, of course, allows you to double the peak voltage swing. And that translates to four times the output power, for the same power supply voltage (since power is proportional to voltage squared.)

So: if we assume the +/- 50V rails allow for an 80-volt peak speaker voltage after 20% voltage losses (in bridge mode, remember), we now have 160 volts peak-to-peak. Convert to RMS, calculate power into an 8 ohm load, and we have - 400 watts!

If this particular class D module actually operates in bridge mode, then I think there is a good chance it actually meets its specifications, provided the power supply can actually deliver half a kilowatt or more of power, while still maintaining, say, +/- 43 volts on the supply rails.

The other possibility is that this module is like those $30, "1000 watt" car amps that you see advertised at car-parts stores. The ones that turn out to have a 5-amp fuse in the positive wire that goes to the car's +12 V power.

-Gnobuddy

[Gnobuddy]

My personal experience has been that their advances in efficiency are pretty effective.

There's no doubt about the efficiency improvements, just look at how tiny the heatsinks are, compared to good old class AB amps.

But Fahey was talking about maximum power output, given a specific power supply voltage. For that calculation, he is entirely correct. It's quite simple, really:

1) The amount of power you can deliver to a speaker depends on the maximum AC voltage you can provide.
2) With no output transformer, the maximum AC voltage the amp can deliver is determined by the power supply rail voltage(s), and (saturation) voltage losses in the output transistors.

For "normal" amplifiers, i.e. not operating in bridge mode, the maximum peak-to-peak output voltage is the total supply voltage minus two output saturation voltages. Typically 3 - 4 V. So Vpp(max) = (Vsupply-4).

Once the peak-to-peak voltage is known, you can easily calculate the RMS power delivered to the speaker. You divide by 2 (to get peak voltage), then by the square root of 2 (to get RMS voltage). Square the result, divide by speaker impedance, and there you are.

More simply, since the square of {2*sqrt(2)} is simply 8, you can say that RMS power = (supply voltage - 4)^2/(8*Rspeaker).

Notice that that we have made no assumptions at all about the amps operating class - the answer is the same whether the amp is class A, AB, or D.

The rules change a bit for bridge-mode amplifiers. As I said earlier, they can put out twice the voltage (peak, peak to peak or RMS, you pick one). That means they deliver four times as much RMS power to the speaker, for the same supply voltage. So the formula changes to: RMS power (bridged amp) = (supply voltage - 4)^2/(2*Rspeaker)

The takeaway: if you use the same power supply voltage(s), and assuming both amps are competently designed, a class D amp will have the same maximum power output as a class AB amp. However, you can use a much smaller heatsink on the class D amp, because of the significantly higher efficiency. So it will probably be smaller, lighter, cheaper, and run cooler. All of which are good things, as long as the Class D amp delivers adequate audio quality for our needs.

-Gnobuddy

[darkfenriz]

Thanks. Thatīs unloaded voltage.
That voltage will drop at least 15% , maybe even 20% under full load (also consider much increased ripple at full load) no matter what kind of amp is pulling that power.
So both a linear and a Class D amp will put out about the same peak voltage on the load (speaker), within a Volt or so, if fed from the same supply .... which is what you will do.

Actually there's a considerable difference here.
1st: Class B amp has an efficiency penalty vs class D of about 30% of total power, which is widely understood
2nd: what isn't so widely understood is that class D amp only consumes the real power it delivers to the speaker (plus some insignificant losses) while class B amp consumes the real power from the PSU based on the apparent power it delivers to the load. In other words class D amp delivering 30V RMS and 4A RMS to a purely reactive load will consume nearly nothing, while class B will typically consume somewhere between 160 and 250 W.

So, the drop may be much lower in class D for real life scenarios.