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Author Topic: New Build Power Amp IC  (Read 3664 times)

Bakeacake08

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New Build Power Amp IC
« on: October 30, 2015, 04:13:28 AM »
A while back I was working on designing a hybrid amp with a tube preamp and SS power amp. I'm finally getting around to finding components for it, and it is my understanding that the TDA2050 is no longer commercially available. I guess my first question is is there In fact no more legitimate source for this part? If not, does anyone know where I could get (preferably) a kit for a power amp IC? My circuit has one 35VDC rail, and I've only been able to find kits for dual-supply operation. I just want it to make a small signal bigger, so I'm not too picky; any run-of-the-mill, standard, boring IC will do just fine. Thanks for any suggestions!

Bakeacake08

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Re: New Build Power Amp IC
« Reply #1 on: October 30, 2015, 04:15:51 AM »
Also, this is more of a SS question and I didn't realize I was in the hybrid section. Feel free to move it if needed.

tonyharker

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Re: New Build Power Amp IC
« Reply #2 on: October 30, 2015, 05:51:02 AM »
The LM1875 can be used instead of the TDA, and there is a single supply circuit in the Datasheet for it.

Enzo

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Re: New Build Power Amp IC
« Reply #3 on: October 30, 2015, 05:23:35 PM »
Five legs may be easier to deal with than twice as many, but what is wrong with going to the LM3886?

Single supply and split supply are not really much different.  For single supply, we run the IC from V+ to ground, with an artificial reference voltage made by two resistors, to set the inputs.  The output will have a DC offset, so we run it through a capacitor.

How did you generate that +35v supply?  What stops you from using the same AC source to make a parallel -35v supply?

Bakeacake08

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Re: New Build Power Amp IC
« Reply #4 on: November 01, 2015, 02:43:59 AM »
35 volts was available locally, so that's where that came from. I'm likely ordering online now, so anything' possible. I went with a single supply because the tube preamp. I couldn't figure out how to do a dual supply that wouldn't leave the tube voltage unreferenced to anything (which intuitively I assume is unstable/dangerous). Is there a way to mix in a tube on a dual supply? If there is, I like the lm3886 kits I've seen. Fairly inexpensive and all the hard parts done. If not, the lm1875 seems easier to veroboard for the single supply.

teemuk

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Re: New Build Power Amp IC
« Reply #5 on: November 01, 2015, 03:22:39 AM »
Datasheet example circuit:


It's principally the same idea described by Enzo, but with a little bit of extra twist.

Supply is unipolar, so chip's "V+" terminal is connected to the positive terminal of the DC power supply (V+) and "V-" terminal is connected to the negative terminal of the power supply (GND/COM). The chip - like many other opamps - works in bipolar-supply fashion and therefore needs its DC offset voltage adjusted to about half of the V+-to-V- voltage. This - at least in simplified opamp theory - provides equal "headroom" for the circuit to operate at both negative and positive voltage excursions. (In practice it will not always apply).

If supply is bipolar (e.g. +15VDC/-15VDC) then "ideally correct" DC offset voltage is 0V.
If supply is unipolar (e.g. +30VDC/GND) then "ideally correct" DC offset voltage is +15VDC.

Note that in both cases V+ terminal's potential referenced to (no pun intented) reference voltage is +15VDC, and V- terminal's potential (with same reference) is -15VDC.

Basically it's only about shifting DC references, while potentials remain the same. The chip only cares about valid supply voltage potential differences provided to its terminals so as long these are not violated you can practically shift the DC references anywhere. Signal path naturally needs to be AC coupled so you need coupling caps in input and output.

In the datasheet application circuit chip is DC biased to these conditions by supplying the input circuit with DC reference. Input is "DC referenced" via resistor RA 75k - not to ground (0V) - but to the "DC offset bias" voltage (which is approximately half of V+). The reference voltage is generated with resistive divider RA91k / RA 100k. CA provides additional filteration to keep offset steady.

The internal ground of LM3886 (pin 7) is similarly hooked - not to power supply ground - but to DC offset bias reference. The twist is that the reference voltage to this terminal is provided via "buffer", here a simple emitter follower. It's basically a voltage regulator, which keeps emitter voltage steady despite of magnitude of current draw by emitter circuit. Base circuit also inherently needs only very low currents to operate.

As long as we can retain ground (reference) currents low, we can keep impedance of the reference voltage source high. This has some benefits, like lesser current draw by the reference circuit itself, and much lower filtering capacitance requirements for the voltage reference node. If ground current draw/sink is higher the reference circuit must be lower impedance. "Buffering" the reference circuit with a (low output Z) current amplifier is common practice to achieve this without sacrificing high impedance of the reference circuit itself.

Yes, it would probably work also in simplified form (sans regulator) referred by Enzo, but I have a hunch that pin 7 draws or sinks considerable currents and the 91K/100K divider circuit would be too high impedance to be compatible. You can probably estimate magnitude of currents within pin 7, and calculate compatible impedance requirements of the resistive divider. I have a hunch that ideal values would prove to be an excessive waste power load, which can be largely reduced by "buffering". The benefits are probably vast enough to support one cheap transistor more in the BOM.

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Quote
I went with a single supply because the tube preamp. I couldn't figure out how to do a dual supply that wouldn't leave the tube voltage unreferenced to anything (which intuitively I assume is unstable/dangerous). Is there a way to mix in a tube on a dual supply?

Well... You don't say much about details of that power supply. If it's single supply of tube circuit + dual supply for rest then both simply have the same ground reference and that's it. If you need to operate the tube circuit from the same dual supply used by rest of the circuit then you simply substitute ground reference of the tube circuit with V- (its technically as low impedance as ground so why not) and use capacitor coupling (which you'd probably do anyway). Not rocket science.
« Last Edit: November 01, 2015, 03:40:19 AM by teemuk »

J M Fahey

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Re: New Build Power Amp IC
« Reply #6 on: November 01, 2015, 02:41:15 PM »
Agree.

You are not telling us what voltage does the tube need nor where it comes from.

Answer that and we'll be able to offer better answers.

As a side note, I use tubes in my hybrid guitar amps, and I use the exact same transformer that powers the regular 100W/4 r amps I have made for decades.

Bakeacake08

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Re: New Build Power Amp IC
« Reply #7 on: November 02, 2015, 01:45:53 AM »
That sounds simple enough. In a normal tube circuit, the ground rail is connected to the earth safety ground. Are there any safety concerns with having it connect instead to the negative rail?


Here is the original circuit I'm working from. I am basically replacing everything after the n-channel JFET with either the lm1875 or lm3886. (I'm also probably going to take out the standby switch and take out one of the LEDs.)

J M Fahey

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Re: New Build Power Amp IC
« Reply #8 on: November 04, 2015, 11:30:38 AM »
Ok, no big deal.
You plug an LM1875 there instead of the TDA2050 and that's it.
If you had designed a PCB, use the same; if using stripboard or simple perfboard, the same, in fact being so simple, *if*  the tube side is built PTP, you can do the same with the 5 leg IC .

My only doubt is about the 12AX7 at such low voltage, curious about actual plate voltage, gain and Vpp signal out capability.

What's the primary use of this hybrid?

Bakeacake08

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Re: New Build Power Amp IC
« Reply #9 on: November 05, 2015, 03:21:09 AM »
Its main purpose is educational--namely to see if I can get it to work. If successful, I'll probably build a cabinet and use it as a practice amp. I did already have a PCB designed for it, so that's good to hear. I'll definitely post results whenever I collect all my parts and throw it together.

 

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