And bam just like that it hit me.
There's a 10k pot on the lower portion to adjust the C offset. That will affect the charge time of the 100uF cap which is likely why there is a DC voltage on the output when the amp is first turned on.
The solution is to replace the two 22k bias resistors in the top portion of the amp with 25k resistors. That way the charge time of the 100uF caps is real close to the same.
Also the idea about how to redo the gain so the 22uF caps do not have a lower resistance path to ground worked well except I goofed and failed to realize I needed a 100k resistor in parallel with the 22k resistor sending the signal to the other amp section.
So the schematic then becomes this.
Output briefly jumps up to 2Vdc then falls back down.
Gonna measure the voltage across the 22uF cap and see what it is and how long it takes to charge. If that causes the jump in voltage it might be possible to use a lower value there.
I did test the amp and there is no noticeable phase shift anymore.
There's a 10k pot on the lower portion to adjust the C offset. That will affect the charge time of the 100uF cap which is likely why there is a DC voltage on the output when the amp is first turned on.
The solution is to replace the two 22k bias resistors in the top portion of the amp with 25k resistors. That way the charge time of the 100uF caps is real close to the same.
Also the idea about how to redo the gain so the 22uF caps do not have a lower resistance path to ground worked well except I goofed and failed to realize I needed a 100k resistor in parallel with the 22k resistor sending the signal to the other amp section.
So the schematic then becomes this.
Output briefly jumps up to 2Vdc then falls back down.
Gonna measure the voltage across the 22uF cap and see what it is and how long it takes to charge. If that causes the jump in voltage it might be possible to use a lower value there.
I did test the amp and there is no noticeable phase shift anymore.
Frequency response test.
Unmodded.
Modded.
Gain dropped slightly which is beneficial for my needs, but the low end frequency response is more flat.
Heard a noise from the amps at about 6kHz. Figure the .47uF ceramic caps are making the noise. So they will need to be replaced by film caps. Will order four when I place a parts order for my heathkit A9C amp.
Unmodded.
Modded.
Gain dropped slightly which is beneficial for my needs, but the low end frequency response is more flat.
Heard a noise from the amps at about 6kHz. Figure the .47uF ceramic caps are making the noise. So they will need to be replaced by film caps. Will order four when I place a parts order for my heathkit A9C amp.
You’ve discovered the piezo tweeter. Can be perfectly normal for ceramic caps. I’ve heard film caps making just as much racket.
Will the noise affect the sound any?
I do need to test the amp with it loaded though.
Gonna use 16 ohms.
Each amp has its own bias supply.
Is there any way to use a common bias supply and use one pot to vary the DC voltage to where as the voltage on one pin 1 goes up the other goes down?
I do need to test the amp with it loaded though.
Gonna use 16 ohms.
Each amp has its own bias supply.
Is there any way to use a common bias supply and use one pot to vary the DC voltage to where as the voltage on one pin 1 goes up the other goes down?
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The spike is very brief, however it was worse before I replaced the two 22k resistors with 35k resistors.
The pair of unmodded boards did similar, however.
Would altering the bias resistors to be like this work better far as setting the DC offset is concerned or is the bias being like it is not causing the voltage to spike like that?
The only thing there is the boards would then not be able to be separated from each other to work on them without desoldering wires.
The pair of unmodded boards did similar, however.
Would altering the bias resistors to be like this work better far as setting the DC offset is concerned or is the bias being like it is not causing the voltage to spike like that?
The only thing there is the boards would then not be able to be separated from each other to work on them without desoldering wires.
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I'm not a "for" bridged amps diyer, so no experience in this regard. Perhaps someone with more experience can comment.
Your (& anyones) unregulated PSU may take a cycle or two of the AC supply to stabilise the dual rails, after the first power ON. That may also be the cause of your power ON 2V thump. Thats why some sort of DC protection , power ON delay, AC failure detection is neccessary
Your (& anyones) unregulated PSU may take a cycle or two of the AC supply to stabilise the dual rails, after the first power ON. That may also be the cause of your power ON 2V thump. Thats why some sort of DC protection , power ON delay, AC failure detection is neccessary
That could work. I've done similar before.
That said I'd likely get a speaker protection board kit as it will also protect the speaker in case the DC offset gets way out of whack during operation.
I'd need to figure a way to drop the voltage from 44V to 12V though.
That said I'd likely get a speaker protection board kit as it will also protect the speaker in case the DC offset gets way out of whack during operation.
I'd need to figure a way to drop the voltage from 44V to 12V though.
You can use two 24V relays in series , one for each ch. , they will work at 44V. Dropping such higher voltages for 12V relay will dissipate more power in the series resistor or regulator
Was thinking about one of these.
https://store.qkits.com/electronic-kits/audio-kits/speaker-protection-kit-mono.html
https://store.qkits.com/electronic-kits/audio-kits/speaker-protection-kit-mono.html
You will have to buy in pairs, 1 for each channel. Look for something with dpdt relay to save cost
Normally with speaker protection kits I use a larger relay anyways for the current handling so I could replace the stock relay with a DPDT relay. That said I might buy the stereo version anyways.
I've got a Lafayette LA-375 amplifier that I use with my dad's TV.
Will two of these amp boards replace that amp?
If not one will two per channel bridged work?
The speaker load is 8 ohms per channel.
According to this the Lafayette is 25WPC.
https://www.worldradiohistory.com/Archive-Catalogs/Lafayette-Catalogs/Lafayette-1973.pdf
If I do this I'm gonna see if either Profusion can match two 2050 chips or I may buy two extra boards and figure out some sort of way I can make a socket for the 2050 so that I can match my own 2050 chips.
Will two of these amp boards replace that amp?
If not one will two per channel bridged work?
The speaker load is 8 ohms per channel.
According to this the Lafayette is 25WPC.
https://www.worldradiohistory.com/Archive-Catalogs/Lafayette-Catalogs/Lafayette-1973.pdf
If I do this I'm gonna see if either Profusion can match two 2050 chips or I may buy two extra boards and figure out some sort of way I can make a socket for the 2050 so that I can match my own 2050 chips.
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1875 will work, similar price, no need to match parts.
Build another, leave that one alone, or sell it, if it is working.
Build another, leave that one alone, or sell it, if it is working.
Matching parts would be done only if I were to bridge two amps.
According to the datasheet.
https://www.st.com/resource/en/datasheet/cd00000131.pdf
Looks like I might need to use two of these amps bridged per channel to get enough power.
I can either get 50 of the 2050 for $35.50 or 26 for $21.84
Might go with 50 and buy two boards and replace the 22k bias resistors with 1% 22k resistors just to make sure the voltage is exactly 1/2 B+ on both amp boards. I'll then figure out some sort of socket I can use for the 2050 and a way to clamp it to a heatsink. I'll then use my RCA WV-98C Senior Volt Ohmyst for matching the chips given it has a center zero scale and a .5Vdc range. Chips that are matched within 20mV or less of 0Vdc will be paired together. I might get some ESD foam so I can stick the chips together in a way that protects the chips from ESD and protects the legs from being bent.
It would be much easier if there's a board for the 2050 that uses dual power supplies as there's a chance that I might not have to match the chips. I'd need a dual secondary 18V AnTek transformer.
Here's a dual supply stereo 2050 PC board. I could maybe modify it to be a bridged mono and I'd do the feedback circuit mod as well given it lowered the low frequency phase shift.
https://www.pcbway.com/project/shar...oard_with_TDA2050_IC_DIY___ELECTRO_INDIA.html
Instead of going bridged is there a way using power transistors to make the 2050 put out more power with keeping the same low distortion numbers?
According to the datasheet.
https://www.st.com/resource/en/datasheet/cd00000131.pdf
Looks like I might need to use two of these amps bridged per channel to get enough power.
I can either get 50 of the 2050 for $35.50 or 26 for $21.84
Might go with 50 and buy two boards and replace the 22k bias resistors with 1% 22k resistors just to make sure the voltage is exactly 1/2 B+ on both amp boards. I'll then figure out some sort of socket I can use for the 2050 and a way to clamp it to a heatsink. I'll then use my RCA WV-98C Senior Volt Ohmyst for matching the chips given it has a center zero scale and a .5Vdc range. Chips that are matched within 20mV or less of 0Vdc will be paired together. I might get some ESD foam so I can stick the chips together in a way that protects the chips from ESD and protects the legs from being bent.
It would be much easier if there's a board for the 2050 that uses dual power supplies as there's a chance that I might not have to match the chips. I'd need a dual secondary 18V AnTek transformer.
Here's a dual supply stereo 2050 PC board. I could maybe modify it to be a bridged mono and I'd do the feedback circuit mod as well given it lowered the low frequency phase shift.
https://www.pcbway.com/project/shar...oard_with_TDA2050_IC_DIY___ELECTRO_INDIA.html
Instead of going bridged is there a way using power transistors to make the 2050 put out more power with keeping the same low distortion numbers?
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These chips are simply limited by the power supply voltage. You may run into current limiting also, but there is no way besides bridging two to get more voltage. If you add extra power transistors in a configuration which has voltage gain there is no point in using a power op amp front end. In those designs a 5532 does just as well if not better.
I saw one circuit where it used a NPN and PNP power transistor for more output power. One transistor was a 2N3055 and the other was whatever the complementary PNP transistor to that is.
That said I know bridging two will for sure get more output power.
Only reason I want to replace the Lafayette is because with another LA-375 that I used at work it seemed lacking on a pair of speakers. That said the one usd with the TV I feed the signal directly to the amp stages using a resistive voltage divider. So it's possible the Lafayette sounds just fine except it does have a speaker coupling cap.
That said I know bridging two will for sure get more output power.
Only reason I want to replace the Lafayette is because with another LA-375 that I used at work it seemed lacking on a pair of speakers. That said the one usd with the TV I feed the signal directly to the amp stages using a resistive voltage divider. So it's possible the Lafayette sounds just fine except it does have a speaker coupling cap.
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Those basic current booster circuits sort of work. It makes it easier for the chip to drive 4 or even 2 ohms. But the distortion figures rise a lot, they are finicky as hell and have absolutely NO fault tolerance.