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Chip Amps Amplifiers based on integrated circuits

LM1875/TDA2050 adding output transistors.
LM1875/TDA2050 adding output transistors.
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Old 4th June 2019, 09:33 AM   #31
analog_sa is offline analog_sa  Europe
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LM1875/TDA2050 adding output transistors.
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Originally Posted by johnnyhou View Post
I also wound another slightly larger one with a single strand and over 200 primary turns, but had to add a 3.3 ohm, 5 watt resistor to make sure the impedance on the primary winding was mor than 4 ohms.
Why would the impedance on the primary side get below 4ohms? You are just driving a pair of emitter followers, right? It is perhaps closer to 4k than to 4R.
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Old 4th June 2019, 12:10 PM   #32
ximikas is offline ximikas
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Ok, i see now you are using common emitter configuration.You can add small resistance to emitters, like 0.1 or 0.33 Ohms, they will stabilyze bias current a little ,also providing some local feedback, increasing linearity. Also what i see ,is high resistance 68ohms with transformer windings, you should add capacitor 1000uf in parallel each 68ohms. This would pass all output from transformer to transistors bases.
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Old 4th June 2019, 02:09 PM   #33
palstanturhin is offline palstanturhin  Finland
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You might also want to check how Nelson made the F6...
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Old 9th June 2019, 10:20 PM   #34
johnnyhou is offline johnnyhou  United States
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Default One sided crossover distortion

After mounting up all my parts and soldering everything up how I like it, I got my scope and started doing some checking and noticed crossover distortion on only one side of the sine wave. The 82 ohm resistors ( I changed from 68 to 82) are .2 ohms difference and the coil windings themselves slightly differ in impedance. The resistor between the two base pins is now at 9.1K instead of 5.6K and the slight buzz at zero volume is gone.
I started trying different low value resistors on the low side with the scope connected until the sine was pure. The overall impedance from one side to the other is different but the sine is clean, and the amp sounds good. The transformer is a 2:1 ratio step down but there are 2 output windings drawing signal from the single input so the power should be about the same, just lower voltage applied to the base of the power transistors. Now that the circuit is complete and I am satisfied, I will add a fan as the heatsinks do get fairly warm. I will post schematics of my exact circuit when I get a chance.
OVER 100 WATTS from a single lm1875 and 2 power transistors from 30-0-30 PSU! Sounds pretty damn good too! I wonder what kind of output could be expected if I put 4 windings on the output of the transformer to drive 4 power transistors in bridge configuration?
Also found another schematic using a transformer for the input, slightly different topology but interesting nonetheless.
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Last edited by johnnyhou; 9th June 2019 at 10:25 PM.
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Old 9th June 2019, 10:33 PM   #35
johnnyhou is offline johnnyhou  United States
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Oh, and thanks ximikas for the suggestions, I tried the capacitors in line with the 68R (now 82R) like you suggested and that did not work at all. It seemed like a good idea but i suppose it just separated the signal's reference to the emitter from the base pin.
Also appreciate the suggestion to look at the F6 by Nelson, search forums does not readily find the thread but I eventually tracked it down.
F6 Illustrated Build Guide
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Old 10th June 2019, 01:15 AM   #36
stocktrader200 is offline stocktrader200  Canada
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another poster in the forum used a decent coupling transformer and PNP outputs (grounded transformer CT) so that 50w could be had from 12v. A 5w head unit amplifier was the input. 12v 3A torroid speaker across the collectors. Seemed to work rather well.

Last edited by stocktrader200; 10th June 2019 at 01:17 AM.
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Old 11th June 2019, 03:31 AM   #37
wg_ski is offline wg_ski  United States
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Once you start throwing driver transformers into the mix - it changes the whole ballgame because now you can get both voltage and current gain out of the added transistor stage. This was very common back in the 70’s, but has fallen out of favor because transformers are expensive compared to chips. A transformer designed for this service cost $20 at Digikey. Making your own certainly saves some bucks - and now you’ve sort of got a vintage design. Emitter degeneration and bias tracking diodes can (and should) be added but you’ll figure that out after it sits there and runs away . The old vintage designs used same sex transistors but the general principle is exactly the same.

In a way, some modern amps have taken a page from some of these old books. A good many professional PA amps use complementary darlingtons, EF3’s or CFPs in common emitter, and drive the whole works directly of a 5532 op amp. It’s cheap, it has a low overall component count, and it works.
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Old 11th June 2019, 06:09 AM   #38
johnnyhou is offline johnnyhou  United States
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I am an automotive technician, but am savvy enough to build computers, repair appliances, and started dabbling in electronics so I am new to the field but was certain there was a way to get a simple op-amp to drive power transistors and get ample power on the output. While nobody responding to my post was real keen on the audio isolation transformer idea, I still had to give it a go. Now that I have the biasing resistors set correctly I wonder if the ferrite ring I started with would sound any better having better geometry and very different frequency response characteristics.
I am loving the idea of using transformers to drive power transistors and have come up with an idea to build a system for my car implementing this technique. I have tons of transformers and can even buy small cores, brand new, for $1 each. I would have to make my own bobbins but not a problem. Building stuff is what I do! lol!
I have noted in other schematics that emitter resistors are necessary when using multiple transistors per rail to drive low impedance loads, but are not necessary when using the single NPN and PNP BJT's.
I used resistors for biasing to cancel the crossover distortion as diodes had no effect. I am guessing that since there is 60 volts across the diode being fully blocked there is no voltage at all allowed to flow in either direction.
My next plan is to wind a transformer with 4 outputs and drive 4 BJT's in bridge mode from a single lm1875. Will be interesting to see the outcome of that venture.

Last edited by johnnyhou; 11th June 2019 at 06:11 AM.
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Old 11th June 2019, 10:09 AM   #39
wg_ski is offline wg_ski  United States
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As drawn you can’t just stick diodes in the base circuit, no. Because of the reverse bias issue. But if you arrange the bias network as a voltage divider from rail to rail you can. Diode, resistor, diode. Then connect the transformer secondaries between the bias tap points and the base. This is the more traditional arrangement from the old 1970’s circuits. You may need to adjust the turns ratio or add a few ohms in series with each diode but that’s easy.

You will never completely get rid of crossover distortion with this circuit. When you get rid of the crossover artifacts with sufficient bias, you will get gm doubling through the region where both transistors are active. So you’re back to having a form of crossover distortion. It is a non issue with emitter followers, but with common emitter it is real. If the entire stage is inside the global feedback loop it becomes negligible. Any time you add gain inside an amp’s feedback loop, the minimum gain for stability goes up so you have to watch that.

If you bridge two of these, build two complete amps and bridge them. It’s much easier to implement the global feedback.
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