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Old 22nd December 2007, 07:16 PM   #1
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Default my new amplifier (beta version)

While working on chip amps based on the LME49810 i had the idea to use an LM4562 to drive a discrete output stage instead of the usual driver ic's.

Once finished my first prototype i'm really impressed by the results, so i have to share the schematic with you.

http://www.telefonica.net/web/escapades/AMPL.mdi

This can be used in two ways: 12W (8 Ohm) class A and 50/100W (8/4 Ohm) class AB bridging two of them.

Notes:

Input cap is ommited since the use of a the inverting part when bridged results in very low input impedance and decoupling must be applied before the buffer. The use of a dc servo is recommended.

Transistor choice is based only on what I have on hand. Replacing with better parts will yeld to better sound.

Bias is class AB and can be changed using the vbe multipliers. The idea is to choose the desired bias current and set it modifying the vbe multipliers to get 0V output with 0V input to allow a better use of the LM4562 output swing.

The schematic ommits decoupling capacitors. Use 2200 uf paralleled with 0.22/1uF polypropylene in current sources, output stage and LM4562 voltage input. Good regulation at LM4562 will give better sound than unregulated/78XX power. Output stage must be unregulated.

Higher current at bias stage will help to get better slew rate at the expense of higher power consumption and heat dissipation. Slew rate at the output stage MUST be higher than slew rate of the opamp to ensure stability.

Current sources at bias stage may be deliverately imbalanced to bias the LM4562 into class A. This can make the driver opamp to always sink or always source depending on what sounds the best
but i havn't done any listening tests. Biasing the opamp into class A will reduce the output swing and it must be done in a way the ompamp is always capable to give enough current to ensure proper slew rate. Calculation is easy and can be done using load impedance and output transistor transconductance and gate capacitance.

The bridged setup should have a 40V/usec slew rate that is fast enough for a 50W/100W amplifier.

Changing the output transistors requires reducing bias voltage since the IRFP ones have really high threshold voltage. Replacing them without altering Vbe multipliers may lead to a deep class A bias that probably will end up with the output stage.

Putting Vbe multipliers in the same heatsink than the output stage will help to give some thermal compensation for transistors that need it (IRFP do really need it). Thermal compensation will not be optimal since MOSFET transistors have different thermal behaviour than BJT ones. It's recommended to choose a largue heatsink and use transistors that do not need to be thermally compensated.

Using a cap in parallel with GS of the lower input capacitance is recommended to help reducing asymetric distortion.

Enjoy bulding it!
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Old 22nd December 2007, 08:47 PM   #2
Nuuk is offline Nuuk  United Kingdom
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What program can we view your file with please?
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Old 22nd December 2007, 09:02 PM   #3
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Quote:
Originally posted by Nuuk
What program can we view your file with please?

Here is in PDF:
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File Type: pdf ampl[1].pdf (29.1 KB, 448 views)
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Old 22nd December 2007, 09:14 PM   #4
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Thanks John!
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Old 22nd December 2007, 10:23 PM   #5
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Sorry guys, i've noticed the schematic misses the 470 Ohms resistors at the gate of the mosfet [Corrected] . Building it without them will lead to catastrophic results.

I've built mine on veroboard in less than an hour and i'm really impressed with the results.

Thanks for exporting it to pdf. I have no pdf printer so i had to use a less common format.
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Old 23rd December 2007, 12:16 PM   #6
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Quote:
Originally posted by ionomolo
Sorry guys, i've noticed the schematic misses the 470 Ohms resistors at the gate of the mosfet [Corrected] .

Thanks for exporting it to pdf. I have no pdf printer so i had to use a less common format.

Here it is:
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File Type: pdf ampl[1].pdf (28.6 KB, 207 views)
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Old 23rd December 2007, 12:27 PM   #7
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Thanks!
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Old 24th December 2007, 01:40 AM   #8
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Interesting design on the current sources.
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Old 24th December 2007, 09:52 AM   #9
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I think the bias stage is OK. Now i'm planning to do a Sziklai output since it does seem to be much better than the simple mosfet one (from audio power amplifier design handbook).

Any advise on transistor choice for best quality audio?
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Old 24th December 2007, 02:02 PM   #10
BWRX is offline BWRX  United States
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Instead of connecting the programming resistor of each current source to the opposite rail, have you thought about connecting just one programming resistor to both current sources?
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