Solid state amp project questions

[Electron Tornado]

I want to build a 1 Watt solid state amp to go in a cigar box, but have a couple of questions.

I found a schematic: http://www.kitsrus.com/pdf/k27.pdf
The data sheet for the IC: http://pdf1.alldatasheet.com/datasheet-pdf/view/19417/PHILIPS/TDA7052.html
The IC I have is listed as a direct substitute for the one in the schematic. The data sheet is here : http://pdf1.alldatasheet.com/datasheet-pdf/view/10677/NTE/NTE7051.html

My questions:

Since the documentation states that this circuit is meant as a building block, I'm guessing I'll need to add a preamp. How would I know if I do indeed need a preamp?

My next question is, how do I determine what specs I need from a preamp? For example, how much do I need to amplify a guitar signal for the next amplifier to use?

[slacker]

I've been playing with these chips a bit recently, you don't need a preamp they will work just fine with guitar level signals. You can just plug guitar straight in if you want, no need for the volume pot and cap. In that particular design you probably want a buffer before it as the input impedance is really too low for guitar. Worth trying it as it is though it might sound fine.

If you do want a preamp for tone shaping or distortion, then maybe try some of the runoffgroove amp type pedal circuits.

[Kesh]

Do you have active pickups? The input impedance of that amp is at most 11K and a passive guitar will probably sound weak and thin through it. Many pedals will function as an adequate preamp though. Or just build an op amp buffer prior to it.

[Mark Hammer]

Almost all small power-amp chips assume they will be receiving a line-level signal.  A guitar-level signal is generally less than line-level, but substantially more than microphone level.  So, yes, the chips WILL "work" without a preamp, but will deliver less output, and generally with less clarity (because of the impedance thing that Kesh notes) if no preamp is used.  The preamp does not HAVE to provide the sort of gain that might bring a voice mic up to line levels.  Indeed, something that adequately buffers the input signal, and provides a gain of 2-4x is probably more than sufficient.

[PRR]

Guitar-amp input sensitivity runs around 20mV (0.020V) for full output. 50-100mV for polite styles (Hawaiian), 2mV for some grinders; 20mV is a safe first guess.

These numbers *may* differ for flea-power amps. I can see it either way. A certain pluck should give a certain loudness *or* a certain pluck should max-out the amp. May depend on playing style too.

So be ready to adjust on test.

Input impedance generally well over 100K; 1Meg is customary.

Figure the chip-amp's sensitivity. You may need to figure the maximum output Volts and find the gain value in the datasheet.

FWIW: I figure the bare chip *without the pot* 100mV and 100K. Yes, that will work, though it won't jump-out on light caresses like a mega-gain amp does.

[tca]

I want to build a 1 Watt solid state amp to go in a cigar box, but have a couple of questions. 

Just build the PUNCH http://www.diale.org/punch.html

1W, higher input impedance and a great sound.

Cheers

[Electron Tornado]

Thanks to everyone for the replies.

I had the schematic of the Punch, and started the build from that. I did notice the impedance on the chip's data sheet, though it didn't seem to make much difference in signal quality whether I ran the guitar through the JFET buffer or around it to the chip.

Paul, I found a discussion on input sensitivity in a data sheet here: http://pdf1.alldatasheet.com/datasheet-pdf/view/8889/NSC/LM3875.html

Input sensitivity is mentioned on page 16. Using equation 8, I get a sensitivity for the NTE7051 of 77mV. Can you point me to a better source to learn how it influences amp design?


After some fiddling, I ended up with the schematic below. The amp sounded really terrible using an MPF102, so I changed to a J201 with much better results. I tried a couple of MPF102s and they would sound OK at the beginning of a note, but then cut in and out until the input signal decreased considerably. I had no problem like that with the J201.

The amp isn't very loud (hey it's only 1 watt), and the volume control has very little clean range before it begins to distort. The 66k resistor was two 33k's in series trying to get some more clean range out of the volume. It helped some. The switch is there to bypass the resistors for hard distortion.

Here's the schematic:

[tca]

Have to try an build the amp with the NTE7051.

Cheers.

[DrBoogey]

All the supply current trough a 100R Resistor 
This will give a lot of "sag" at full power.
If the amp draws 50mA, there is a voltagedrop of 5 Volts.
I would remove that one.

Cheers

[Kesh]

Though a little voltage sag is a good thing on guitar amps, for compressed sound. Maybe 22 ohm.

[Electron Tornado]

Though a little voltage sag is a good thing on guitar amps, for compressed sound. Maybe 22 ohm.

I took out the resistor completely. The amp sounds much better. An MPF102 actually sounds good in there now.

Hmmm...makes me kind of feel like one of those women in that Prego commercial. 

[PRR]

> Input sensitivity .... .... influences amp design?

There's several ways to specify it. Generally: how much voltage (midband signal) is needed at the input to produce maximum "undistorted" output.

A guitar can deliver over 200mV. But often we do not want to strum that hard. Also the strings fade. In many styles we want to hear the amplifier distortion products fade, distortion down to quite small input signals.

My benchmark for ~~1959 Fender is 20mV input will make "full" output. (Volume at 10, MID fairly high up.)

If you go back to swing/hawaiian/lap guitar amps, which generally did not do loud leads or were never supposed to make rude noises, 50mV was more typical.

If you come to things like a SRV 5150, rock-grinders, 5mV or 2mV input will bring them to FULL output.

If you run a old-standard 20mV amp with an LPB booster in front you have about 3mV sensitivity. (So the 5150 kinda has built-in boost.)

High sensitivity (full output with small input voltages) lets you play with less effort, or lets you stay overloaded longer as the strings fade.

The flip side is that high sensitivity also brings up the universal hiss.

Remember that sensitivity (defined this way) is for VOLume (gain etc) at MAX. Many players set the knobs less than 10 (OTOH some go to 11). Some of the late Fender Tweeds were probably meant to play "nice" around 5 or 6, so 40mV or even 200mV sensitivity.

> I get a sensitivity for the NTE7051 of 77mV

Beats playing unplugged. Probably plenty-enough for plinking around the house.

Some players (some pickups, some pedalboards) thrive with quite modest sensitivity. It has been reported that players allowed to try Angus AC/DC Young's rig find it quite tame, in their hands. Suggesting that Angus works hard. Or there's a hidden switch; but it's also said that he recorded directly into the console, which is often 100mV sensitivity.

Try your 77mV sensitivity. Try an LPB in front. Try a TL072 rigged for gain of 4 in front. You may find that high sensitivity (small mV for full output) seems impressive but is "too much" for most work. That's why amps have knobs.

BTW: that 66K:10K divider switch gives you 580mV sensitivity. You are never going to fill the stadium with a light touch that way. OTOH it may be difficult to "play SOFT!" without some such sensitivity reduction.

> All the supply current trough a 100R Resistor

Yeah, well, that would be reasonable for a NON-power amplifier such as a pedal. When the load sucks big power (such as a Power Amplifier) we scale that accordingly.

The math here gets tedious. The totem-pole audio amp presents a load to the supply like a resistor 6 times the speaker impedance. For 8 ohms, figure 50 ohm effective load on the supply. But the 7051 (most car-chips) is a bridge. Makes 4 times the power for a given 12V, also acts like 4 times lower. So 12 ohm effective load on the supply. For a "no"-sag filter we want less than 10% drop. 1.33 ohms max power filter dropper.

However Power chips are usually designed to eat garbage. They take raw car battery power, alternator whine, power-seat load-dumps, etc, and deliver clean soundz. Generally you don't filter the power to a power amplifier.

The preamp may need filtering. Hi-gain preamps made without chips usually do. Even with chips you often need a small amount of cleaned-up bias voltage. In this case the FET source-follower adds no gain, has good supply rejection, and anyway this rig won't make tiny signals big. Unless you use a punked-out wall-wart, it probably needs no filtering. (A chunky cap across the rails is always a good idea, and for a bridged-amp 2,000uFd is not too big.)

[Electron Tornado]

Thanks for the reply, Paul. Let's see if I understand a little better:

> Input sensitivity .... .... influences amp design?

There's several ways to specify it. Generally: how much voltage (midband signal) is needed at the input to produce maximum "undistorted" output.


My benchmark for ~~1959 Fender is 20mV input will make "full" output. (Volume at 10, MID fairly high up.)

So what that means is, that for an amplifier to produce it's maximum clean output with the volume at 10, it requires X volts. For your 1959 Fender, that requirement is 20mV. Any less than that, and the amp isn't giving it's full output, and any more than that 20mV will result in a degree of distortion in the output. Correct?

If you run a old-standard 20mV amp with an LPB booster in front you have about 3mV sensitivity. (So the 5150 kinda has built-in boost.)

Is that 3mV the sensitivity of the LPB itself, or does it contribute to an overall sensitivity of the entire amplifier?

[Electron Tornado]

> I get a sensitivity for the NTE7051 of 77mV

Beats playing unplugged. Probably plenty-enough for plinking around the house.

BTW: that 66K:10K divider switch gives you 580mV sensitivity. You are never going to fill the stadium with a light touch that way. OTOH it may be difficult to "play SOFT!" without some such sensitivity reduction.

Yes, with that 100R resistor gone, it's quite good for bedroom or living room playing.

The Punch amp didn't have a volume control. The 66k resistance was added to keep the output clean as the volume was turned up. With that resistor bypassed, the output has some distortion, even at lower volume levels.

That 77mV sensitivity was for the chip only. How is the sensitivity calculated for the whole circuit?


I think I also need to spend some time learning about power supply basics.

[tca]

The Punch amp didn't have a volume control.

If you want the Punch with a volume pot try the TDA7052A chip (http://www.diystompboxes.com/smfforum/index.php?topic=99407.msg880683).

Cheers.

[PRR]

> 3mV the sensitivity of the LPB itself

No, ideally the Linear "Power Booster" never distorts, just adds clean gain to push your amp harder.

(Of course it WILL distort, and on hot pickups probably does.)

> How is the sensitivity calculated for the whole circuit?

What is the gain of the JFET stage? About 1. Perhaps 0.9. What is the gain (loss) of the pot and resistors when pot is full-up? Near 1, say 0.99. If a gain is 0.9 then you need 1.1 units signal in to get 1 unit out. Or 77mV*1.1*1.01= 85.5mV. Close enough to 77mV.

> With that resistor bypassed, the output has some distortion, even at lower volume levels.

That's not right. 

The un-biased JFET does not have a lot of headroom over a guitar's signal, but adding the 66K won't change that enuff to notice. Check JFET voltages. Try a JFET with a higher threshold voltage.

[Electron Tornado]

> How is the sensitivity calculated for the whole circuit?

What is the gain of the JFET stage? About 1. Perhaps 0.9. What is the gain (loss) of the pot and resistors when pot is full-up? Near 1, say 0.99. If a gain is 0.9 then you need 1.1 units signal in to get 1 unit out. Or 77mV*1.1*1.01= 85.5mV. Close enough to 77mV.

> With that resistor bypassed, the output has some distortion, even at lower volume levels.

That's not right. 

The un-biased JFET does not have a lot of headroom over a guitar's signal, but adding the 66K won't change that enuff to notice. Check JFET voltages. Try a JFET with a higher threshold voltage.

Here are the voltages. Supply voltage is 9V from an adapter.

V(Drain-Source) = 5.7V
V(Drain-Gate) = 0.8V
V(Source-Gate) = 0.8V

[tca]

My suggestion: remove the J201 (low Vth=-0.83V) and replace with the MPF102 (Vth= -2.34V), remove the volume pot and the 10k input resistor. What guitar and pickups are you using?

ah, just found your problem. If that IC is a replacement for the TDA5072 it should have a  5k to 10k input resistor to ground.If I recall it correctly a bigger value resistor will make the IC break  to early.



Probably if you want to keep this IC the best way is to connect the pot cursor to the output of the buffer, the other terminal to ground and the upper terminal to the IC.

Cheers.

[Electron Tornado]

tca:

I guess I should post a revised schematic. When I removed the 100R resistor, I tried an MPF102 again and it worked well, so that's what I have in there now.

Great idea on how to place the volume pot! I'll give it a try.

[PRR]

> V(Drain-Source) = 5.7V
> V(Drain-Gate) = 0.8V
> V(Source-Gate) = 0.8V

Hmmm, I often see pin voltages with respect to Ground.

But the data is there, just work for it:

D = 9V
S = 9V-5.7V= 3.3V
G =  9V-0.8V= 8.2V

Well, that don't add up, that can't be right.