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Old 19th July 2005, 04:01 PM   #1
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Default Front end for sub amp

I have prototyped a small utility amp like the one attached (38v supply, some of the values in the actual prototype differ from those in the diagram but it gives an idea of the topology to which I will be connecting). I want to use it for a subwoofer amplifier. I have a bunch of small signal bi-polars lying about so I want to make a summing low pass front end to operate with speaker level inputs.

My next post will show the idea that I have in mind. I would like to get your input on the basic plan and some guidance on a few details.

mike
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Old 19th July 2005, 04:11 PM   #2
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Default The front end idea

This is the frontend I have in mind.

R7 represents the input impedence to the power amp. R5 and R6 represent a pot set to relatively high attenuation not actual discrete resistors. C1 and C3 are set to give 12dB/8va at about 70Hz.

Is this basic layout a sound idea? For the second channel would I duplicate R4, R5, R6 and C2 and join the two inputs at the bas of Q1?

I didn't plan to include variable cutoff frequency but I suppose I could if it is simple enough to do.

My plan is to add the frontend circuit to the power amp and connect the B speaker connection on my reciever to the inputs of the subamp and use the sub amp to drive a relatively efficient 15" IB sub.

mike
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Old 19th July 2005, 04:12 PM   #3
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Hallo
You need a resistor at input series to C1 to have fixed closed loop gain.
Also I'd add some small resistors for better termal stability and maybe Vbe multiplier unless you want to drive output in pure class B.
regards

edit: the above refers to post1
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Old 19th July 2005, 04:28 PM   #4
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Thanks for the quick reply.

Quote:
You need a resistor at input series to C1 to have fixed closed loop gain.
Would 10k or so be adequate?

Quote:
Also I'd add some small resistors for better termal stability and maybe Vbe multiplier unless you want to drive output in pure class B.
I am not sure what you mean by Vbe multiplier. Small resistors where?

In the prototype I started with two diodes instead of Rb but the voltage drop on them was too large and the amp turned into a space heater so I switched to a resister on the perf board to get it working. I would like to switch back if I can come up with the appropriate diodes that will give a bit less bias voltage. I had used rectifiers but I think I need to try signal diodes instead.

Even with just the resistors in place I was able to play quite loudly for almost an hour without the amp getting hot but I may actually be biased in class B.

mike
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Old 19th July 2005, 05:47 PM   #5
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The gain of your amp will be R2||R3 divided by input resistor.
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Old 20th July 2005, 01:53 AM   #6
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Ahhh. OK, I see. Thankyou.

mike
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Old 21st July 2005, 03:00 AM   #7
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Dark,

I had some time to study your mods a little bit more and I really like them. The bias transistor looks like a very nice trick as it allows for adjustment of bias current (not possible with simple diode compensation) and if my analysis is correct an open circuit failure of the bias pot wiper would decrease the bias voltage throwing the amp into a very safe class C operation. The resistors in the output circuit will not have a great effect on the current through R7 and R8 so those values should not need to be changed.

Could you explain in more detail how the .1 and .22 ohm resistor work in stabilizing the bias? They sure are nice for measuring bias current.

I appreciate your patience.

mike

P.S. I note no one commented on the input circuit. Does that mean that it is generally sound? I am sure that some of the values will need to be tweaked. I am considering using DIP sockets to insert the R-C filtering components so that the crossover can be easily changed. I will also probably make the input circuit switchable so that it can be switched out for use as a full (audio) band utility amp.
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Old 21st July 2005, 10:10 PM   #8
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Yes, your analysis is very acuurate, for these reasons this biasing circuit has become a standard.
The small resistors give emitter feedback. This is usefull for linearity adn termal stability. As emitter follower has a positive thermal coefficient, the bias current is higher for higher temperatures. For example 0.65V bese-emitter drop may cause 20mA for 20deg. and 200mA for 90deg. The resistors provide better thermal stability because of voltage drop proportional to current and thus lower drop on base-emitter junction.

Your input circuit is simple and useful 12dB/oct filter, but I see some potential problems.
First is rail noise, with R2 and R3 you set biasing of a transistor, but nothing stops the rail noise from mixing with input signal. Try regulating the rail, 40V is quite high, you don't need that high. Simple lm7818 (or higher) or lm317 (with resistor divider ) should do.
Second is DC through a pot. R2, R3 , R4 and a pot make voltage divider, so there is some DC through a pot, which can be very noisy while adjusting. Try a capacitor series with R4.
Third is low input impedance. The input sees less than 1k resistance to the ground, try bigger pot, like 47k with smaller input cap. 1000uF is hard to be of high quality, best are some polipropylene non-polar of value less than 47uF.
Hope this helps
regards
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Old 21st July 2005, 11:42 PM   #9
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One solution is to modify the JLH and make it a class AB amp. I did so and posted a schematic of it a while ago. It sounded very well in real life too.

Let me know if you would like to see it.
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Old 22nd July 2005, 03:35 AM   #10
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Quote:
Your input circuit is simple and useful 12dB/oct filter, but I see some potential problems.
First is rail noise, with R2 and R3 you set biasing of a transistor, but nothing stops the rail noise from mixing with input signal. Try regulating the rail, 40V is quite high, you don't need that high. Simple lm7818 (or higher) or lm317 (with resistor divider ) should do.
Second is DC through a pot. R2, R3 , R4 and a pot make voltage divider, so there is some DC through a pot, which can be very noisy while adjusting. Try a capacitor series with R4.
Third is low input impedance. The input sees less than 1k resistance to the ground, try bigger pot, like 47k with smaller input cap. 1000uF is hard to be of high quality, best are some polipropylene non-polar of value less than 47uF.
Hope this helps
Thanks for the detailed response. It is very helpful. I was I must admit a little careless with the drawing of the input circuit in the following ways.

1. I did some cut and paste so the value of the input capacitor was a random value from something else.

2. The 40V was just an approximation of what I remembered the power supply of the power amp section to be. It turns out that it is about 38V. I intend to just use the same supply.

3. I did not show any PS decoupling. I was planning on an R-C decoupling network but your suggestion of an IC regulator makes a lot of sense. Sounds like a good idea. I think I might even have an LM317 in my parts bin.

Is it a good idea to put a cap between ground and the output of the voltage regulator to shunt any noise?

mike
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