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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 23rd May 2019, 03:50 AM   #21
wg_ski is offline wg_ski  United States
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Join Date: Oct 2007
You’re making a chip amp way harder than it has to be. The original circuit you posted is the only one of the bunch that is practical, then the Rbe value needs to be 1.5 to 2 ohms to get the transistors to turn on at all.then all it can do is boost the current capability - you are still limited by the supply. And the circuit can be finicky, and in ALL cases gives up the built in overload protections that the chip amp is supposed to provide. Short the speaker wires and kapowie, you buy new TIP’s and maybe the chip.

If you need external transistors to provide voltage AND current gain you really don’t want a power op amp in the front end. There are several topologies using two voltage/current gain stages driven by a normal op amp like the 5532 that are a whole lot easier to get working than what you just proposed. Research the QSC circuits for ideas. And they do make versions that don’t use flying rails.
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Old 24th May 2019, 07:33 AM   #22
johnnyhou is offline johnnyhou  United States
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Well guys, I wound a ferrite ring about 1" in diameter with a length of 4 stranded magnet wire for an output transformer just to see if it would work. I connected the end of 1 conductor to the start of another giving double the turns for the input, coming from pin 4 of the LM1875. Other end to ground. Resistance of the full length was just over 4 ohms so okay there. The other 2 conductors I connected to the base and emitter of my 2 transistors. I had to crank the input volume up almost all the way to get full sound from the output via the collectors to my speakers (wired parallel for 4 ohms) pretty damn loud and surprisingly punchy too. I thought I would lose bass response using ferrite instead of laminated steel but the setup actually works! It could sound better but I suppose I wound mimimal turns to get a signal out, and the voltage output is 1/2 of the input but it is something. I have a small steel lamination transformer I may wind up with a more appropriate number of turns later but this has been an interesting venture thus far.
I still plan to try a single ended circuit with single power transistor but I would have to connect one speaker connection to positive rail and the other to the collector of NPN power transistor. I may try that scheme tomorrow.
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Old 2nd June 2019, 02:10 AM   #23
johnnyhou is offline johnnyhou  United States
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UPDATE:
I upped my voltage on my PSU to 30-0-30 and wound a couple output transformers to isolate the feed to the power transistors. While this method does get output to the speaker up to about 100 watts, the volume of the source must be high enough to deliver a strong signal to the base of the transistors, and even then the output is heavily distorted. I tried adding a resistor or capacitor in line to the base of the transistors hoping that might clean things up a bit but nothing satisfactory there. I could bridge a pair of lm1875 chips to increase voltage into the isolation audio transformer in hopes of getting a cleaner signal to the base of the power transistor but I think the real issue here is crossover distortion.
While I had thoughts about using a single supply and a single power transistor in common emitter configuration to boost the voltage and current to the speaker, I foresee some issues there considering that the input signal sweeps both positive and negative voltages from a common ground. I am not familiar with the exact functions of these chips in single supply configuration, though I did get my O-scope in the mail yesterday. Need to find a power supply for that thing and do some investigating I suppose.
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Old 2nd June 2019, 07:05 AM   #24
ximikas is offline ximikas
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Hi , with isolation transformers for output transistors you will hardly get proper DC offset at output ,transformer didn't pass DC , so transistors may even amplify different positive and negative signal polarity especially if no feedback used.
Frequency response will be not good too .
To avoid crossover distortion ,you need to partially open transistors ,then they will amplify even small signals ,diodes and resistors can be used for that. Also need to know about small input resistance of bipolar transistors , so your transformers secondary winding should be wound with heavier wire than primary and have less windings .
I have read about output transformers technology ,when multiple windings are wound at once ,and parts of them connected in series ,making primary ,other connected in parallel ,increasing output current and reducing output resistance.
When you will have oscilloscope ,connect a audio signal generator to input ,pc or even phone with some app can be used to generate 1khz test signal ,load amplifier with dummy load resistor, high power with heatsink ,and watch output with oscilloscope ,also look at power supply voltage amplitude ,maybe output at given supply is already reaching maximum possible minus transistor saturation voltage.
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Old 4th June 2019, 01:49 AM   #25
johnnyhou is offline johnnyhou  United States
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Thanks ximikas, I have looked at using diodes to offset the crossover junction but I am thinking that the two separate conductors from the isolation transformer may be preventing any bias corrrection? since the opposite end of each conductor is tied to the respective emitters there is 60 volts directly across the two base pins, and it seems the diodes should bleed a measured amount of voltage in reverse bias mode. I have used a battery to measure the amount of voltage bleeding through diodes in reverse bias direction, and have seen a measured amount of difference in them as they have been used and sometimes abused with excessive heat from a soldering iron or other heat dissipation. (from .04 volts up to .8 or .9 volts on occasion, and sometimed full voltage of the battery as the diode is no good.) Seems like I should find a single diode with a high measured reverse bias voltage as I have noted that more diodes actually DECREASES total voltage of bias offset. I am still toying with the circuit and had a full sound using a couple resistors but of course they got quite hot rather quickly. At any rate I am sure there is a viable outcome of success with this odd topology: chip-audio transformer-power transistors. When I get the circuit to operate properly I will share what I have learned from the venture here, and thanks to all for the input and helpful insight!
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Old 4th June 2019, 03:20 AM   #26
johnnyhou is offline johnnyhou  United States
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Okay, I have it working fairly well now, used 68R2W resistors from the rails to the leading ends of the 2 secondaries on my output transformer, and a 5.6k 5W resistor between the 2 base pin connections at the other ends of those secondaries. There is a bit of overlap and perhaps the quiescent current is still a bit high but the sound is full and good all the way to zero volume, which at that point there is a very slight hum. I can still play with the circuit a bit but at this point I will say I am quite satisfied with the outcome thus far. Just a couple tweaks away from the build I have been striving for with the parts I have up to my ears. Output at speaker is 102 watts RMS (clipping) using a single pair of output transistors.
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Old 4th June 2019, 04:18 AM   #27
asuslover is offline asuslover  Romania
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LM1875/TDA2050 adding output transistors.
102 watts with one pair of transistors? At +-30v dc?
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Old 4th June 2019, 06:28 AM   #28
johnnyhou is offline johnnyhou  United States
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I used my multimeter measuring AC at the speaker connection with speaker connected. Had pillows over the speakers to dampen the noise but still disturbed the neighbors for a minute. Measured 20v and the impedance of the load disconnected after the test was 3.9 ohms. 20^2/3.9 is 102.564, am I doing this right? I did not use the hold function which would hold the highest value correct?
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Old 4th June 2019, 07:20 AM   #29
ximikas is offline ximikas
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Paint a picture , how your transistors and transformers are connected. Biasing methods are diferent if using common emitter and emiter follower configuration. I measure power the same way, but need to look with oscilloscope,to ensure no clipping occurs yet, and then measure ac voltage at output. Better use lower frequency , not all voltmetters and dmm measures ac voltage accurately ,if frequency is more than 1khz. Clipped power output has no practical reason, you will not like that distorted output and not listed at that level.
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Old 4th June 2019, 09:20 AM   #30
johnnyhou is offline johnnyhou  United States
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I had to use multiple strands of very thin magnet wire wound in parallel to pass 3 amperes, lay into the window flat, and provide enough turns to saturate the very small core on the primaries. 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. I wound the secondaries both at the same time (bifilar) and made sure the beginnings went to the rails, and the endings to the base of the 2 transistors.
There are many variables that can be altered to achieve better performance with this method for utilizing a simple chip amp to drive a set of output transistors. Different winding ratios, higher impedance on the primaries would allow a higher voltage and less strain on the chip, while using fewer turns for the secondaries stepping down the voltages applied to the base of the transistors. Using diodes for biasing the transistors does not seem to work at all probably due to the 2 driving circuits being completely isolated from one another. The resistors at the rails provide further isolation from the DC voltages thereby allowing a much higher value resistor between the 2 base pins to offset the zero point of the 2 transistors.
When I have some time to use my scope to get a better idea of those transitions I will get some photos. I did at least find a 9 volt wallwart so I can finally actually start using my scope. (Now that I am pretty much finished.)
Anyhow, enjoy!
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