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Old 21st May 2012, 04:13 AM   #1
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Default Simple amp circuit obsevations

So over the last two days I put together the three circuits shown below. I really want to start getting a better understanding of the basics of solid state power amps. I built them one at a time, and when each one was finished I listened to it for about an hour, really trying to listen to the sound of it. I was hoping to ask a couple of questions. Here is what I came up with:

Little Amp1 - It sounded good. Nice lows and highs, maybe just a slight hint of distortion. Sounded like the middle was very slightly scooped out. It kind of sounded like all of the consumer amps I have owned over the years, low priced, integrated amps. Nothing special, just decent.

Question - I'm not sure what class this amp is biased at. ( first novice question ).

Little Amp 2 - A little more distortion, completely different sound. Less low end but more pronounced from the upper low end to the high end. More grainy. Not smooth from top to bottom like amp1. ( this is the first time I've tried to explain the sound of something ). It kind of started to grow on me the more I listened.

Little Amp 3 - More distortion. Had a hard time biasing the 1/2 Vcc. Just took a while to finally settle down. After that it never really did sound right. It did sound more like amp 2 than amp 1 though.

I used fairly good parts for these amps. The transistors were what was spec'ed on the schematic. The resistors were 1% metal film 1/2 and 1 watt. Caps were all newer. The power supply was an adjustable, regulated supply.

I know I could just build a JLH, or a Pass amp as a good starting amp, in fact I just finished a little solid state, p to p stereo amp for my daughter, but it was well documented and easy to build. I want to understand a little better what happens and how the sound improves when I add, (or subtract) parts to a circuit.

So starting with Little Amp 1, I would like a couple of suggestions on adding two or three parts, one at a time, so I can listen to the difference, (improvement) it makes. I did order Bob Cordell's book but I'm itching to get started now. Thanks.

Tom
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File Type: jpg Little Amp 1.jpg (76.1 KB, 649 views)
File Type: jpg Little Amp 2.jpg (66.1 KB, 615 views)
File Type: jpg Little Amp 3.jpg (53.1 KB, 602 views)
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Old 21st May 2012, 07:32 AM   #2
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Global feedback suggestion.
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Old 21st May 2012, 08:25 AM   #3
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Little amp #1: Assuming 8 ohm load, you get 25 mW (order of magnitude) in class A, from the 46mA quiescent current shown on the schematic (~80mA peak output). After that it would be pretty nasty I think. I expect high distortion even in the class A range.

It's basically the JLH simple class A output stage without global negative feedback. Because of that, you've needed to remove the bootstrap from R4 to set the gain. Increasing the current is my first suggestion.

After that, maybe restore the R4 bootstrap and apply global negative feedback.
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Old 21st May 2012, 08:49 AM   #4
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by tom42107 View Post
So starting with Little Amp 1, I would like a couple of suggestions on adding two or three parts, one at a time....
Little Amp 1:
  • Replacing R4 with a potentiometer would be a good start, so you can adjust the quiescent current (preferably higher).
  • The next step would be to try bootstrapping it as in the first pic below. Making C4 a lot bigger than 220uF is probably a good idea. R5 could probably be bigger too.

Little Amp2:
  • Reducing R3 from 50R to say 10R and increasing the quiescent current would allow higher output and probably lower distortion at the same output level. It will also improve the loudspeaker damping, which may sound better or worse.
  • Removing C4 will result in less gain, but also much lower distortion.
  • Putting an emitter follower in front of Q1, as in the second pic below, will make the amp much easier to drive.

Little Amp3:
  • As with Little Amp 1, try reducing R2 and increasing the quiescent current.
  • Putting a small resistor in series with the MOSFET source will improve linearity.
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File Type: jpg Little Amp 2b.JPG (34.9 KB, 531 views)
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Old 21st May 2012, 12:27 PM   #5
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Improved #1 circuit - Simple Amplifier
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Old 25th May 2012, 06:04 PM   #6
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Quote:
Originally Posted by steveh49 View Post
Little amp #1: maybe restore the R4 bootstrap and apply global negative feedback.
Quote:
Originally Posted by godfrey View Post
Little Amp 1:
  • Replacing R4 with a potentiometer would be a good start, so you can adjust the quiescent current (preferably higher).
  • The next step would be to try bootstrapping it as in the first pic below. Making C4 a lot bigger than 220uF is probably a good idea. R5 could probably be bigger too.

Quote:
Originally Posted by Lazy Cat View Post
Improved #1 circuit - Simple Amplifier
As far as Little Amp 1, I added the bootstrap and built the amp as per Godfrey's suggestion. Result:
1. It was really hard to drive. I'm using my laptop output and I couldn't get it loud enough. How can I make it easier to drive?
2. Now that I have the bias pot and the bootstrap pot, I'm not sure how to adjust them. I'm assuming the bias pot still adjusts the quiescent current at the collector of Q3.

I also built Lazy Cat's amp. It started out really distorted, (overdriven) so I changed out BC550 for a 2N3904. That really seemed to help a lot. Should mention that I'm using 2N3055's for output transistors. Also, how could I add a high pass filter to it? Am I still adjusting half of Vcc at collector of Q3? Thanks.

Tom
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Old 25th May 2012, 06:39 PM   #7
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by tom42107 View Post
As far as Little Amp 1, I added the bootstrap and built the amp as per Godfrey's suggestion. Result:
1. It was really hard to drive. I'm using my laptop output and I couldn't get it loud enough. How can I make it easier to drive?
2. Now that I have the bias pot and the bootstrap pot, I'm not sure how to adjust them. I'm assuming the bias pot still adjusts the quiescent current at the collector of Q3.
Pot 1 adjusts the DC voltage at the junction between Q2 and Q3. That should be set to about half the supply voltage. It will work better if you take out the 22K resistor and connect the left side of the pot to ground, like in the pic below.

Pot 2 adjusts the quiescent current of Q2 and Q3. You can check that by measuring the voltage across R6.
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Old 25th May 2012, 08:08 PM   #8
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I'll try that tonight. Novice question #2, what voltage am I looking for across R6? Will this determine whether this amp is biased class A, AB or B? Also wanted to say thanks for your help and expertise. To Lazy Cat and steveh49, thanks too.

Tom

Last edited by tom42107; 25th May 2012 at 08:31 PM.
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Old 26th May 2012, 07:39 AM   #9
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Quote:
Originally Posted by godfrey View Post
Pot 1 adjusts the DC voltage at the junction between Q2 and Q3... Pot 2 adjusts the quiescent current of Q2 and Q3. You can check that by measuring the voltage across R6.
Other way around. Pot 1 sets the currents and Pot 2 sets the output voltage (for a given value of P1).

Tom, this amplifier has to be biased in class A. It will be awful once you go outside that. Idle current of Q2 & Q3 = voltage across R6 divided by R6 (Ohm's law V=IR). Work out how much output power you want and then set the idle current to about 2/3 the max output current. Depending on how much power you want, you might need to adjust the Q1 current as well by changing R5.

Have you drawn the schematic in a simulator? Running the sim would teach you a lot.

Are you using headphones? The output capacitor seems sized for a higher impedance load than typical speakers.
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Old 26th May 2012, 07:57 AM   #10
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LC, wat proggy do u draw ur schemos in ?
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