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Sound Quality Vs. Measurements
Sound Quality Vs. Measurements
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Old 4th August 2013, 01:01 PM   #10161
nigel pearson is offline nigel pearson  United Kingdom
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Well I made a start . It works and for a 15 inch bass it ticks the excellent box already . At 10 kHz it is OK and 22 kHz it is not .

It uses every bad idea I could muster and alas lives up to expectations .

It is stable . MPSA 42 and 68 pF . A very cautious start so as not to have smoke . 5 transistors total . 80 watts .

LTP current is a modest 1 mA and is using a tail resistor . Although calculated balance it must be about right as it has very little second harmonic . 9 mV offset . Gain is 48 . Inputs are 2SA970 .

I will work on to get 22 kHz right . Other than that a qualified success . Hum is rubbish . Looks a mess .

Correction . Just redid hum - 90 dB ref 1 watt 150 Hz ( worst ) . Not bad for a start and remembering 33.6 db gain . The gain will come down as step one to lower distortion .

Last edited by nigel pearson; 4th August 2013 at 01:10 PM.
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Old 4th August 2013, 01:04 PM   #10162
vacuphile is offline vacuphile  Netherlands
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That's an amp for a Mad Max movie :-)
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Old 4th August 2013, 01:13 PM   #10163
Max Headroom is offline Max Headroom  Australia
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That's an amp for a Mad Max movie :-)
"Let's face it Max, you're a mess".

Dan.
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Old 4th August 2013, 01:17 PM   #10164
nigel pearson is offline nigel pearson  United Kingdom
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Thanks guys .

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Old 4th August 2013, 01:34 PM   #10165
dvv is offline dvv  Serbia
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"Let's face it Max, you're a mess".

Dan.
He ain't no mess, he's Nige Pearson!
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Old 4th August 2013, 01:50 PM   #10166
Max Headroom is offline Max Headroom  Australia
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He ain't no mess, he's Nige Pearson!
That was a quote from the original Mad Max movie .

Dan.
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Old 4th August 2013, 02:05 PM   #10167
dvv is offline dvv  Serbia
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That was a quote from the original Mad Max movie .

Dan.
I am well aware of that, my comment was a paraphrase of an old Hollies hit "He Ain't Heavy, He's My Brother".

But, as ever, it appears I'm more ancient than anybody else online.
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Old 5th August 2013, 08:42 AM   #10168
nigel pearson is offline nigel pearson  United Kingdom
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As said this is not intended to be a full frequency amplifier ( the Mad Max as it will now be called ) . The 10 kHz is now good enough to need to use the Audio Precision . The 22 kHz is 0.3 % second harmonic is not too bad . Looking at it up and down the power range nothing bad happens which says it is just VAS as far as I can see . 2 pole compensation and going down to 15 pF was not the answer .

This amp goes against the grain with me . The VAS to LTP driven by 1K6 is pure trans-conductance ( u = 100 ) . I have a 2SC2910 of 3 times the gain . That should take it inside my rules . BTW . At LTP 1mA re = 50 . That is a minor reason to stick with it . If someone would prefer to use MPSA 42 I would raise LTP to 3 mA . That makes the collector resistor 510 R . I would add 39 R emitter resistors to the LTP . Move VAS cap to 68 pF at the start .

Everything was done to be " wrong " here . Wrong with OK maths . If I can get the 22 kHz distortion down I think I might use as a main amp for a while . I have a hunch it will be very detailed and sweet . As far as I can tell no obvious crossover distortion problems at any power level into noise . 220R was the resistor nearest to the bias point . Just think, one resistor . Bias failure can not happen .
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Old 5th August 2013, 10:11 AM   #10169
nigel pearson is offline nigel pearson  United Kingdom
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By small adjustments the 2SC2910 works better . MPSA 42 did a good job . Notice how a CCS I fitted was slightly worse ( JFET current diode ) .
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Old 5th August 2013, 01:27 PM   #10170
xrk971 is online now xrk971  United States
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Sound Quality Vs. Measurements
Quote:
Originally Posted by nigel pearson View Post
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Well I made a start . It works and for a 15 inch bass it ticks the excellent box already . At 10 kHz it is OK and 22 kHz it is not .

It uses every bad idea I could muster and alas lives up to expectations .
Nice work Nigel! I love point-to-point aka dead-bug style circuits. Really, they are the epitome of efficient network wiring and the same way neurons in our brains are set up. They may look fugly, but they can work really well.

Here is something I did with a TPA3118D2 class-D chip and very small SMD dead-bug style on foam core board: http://www.diyaudio.com/forums/class...ml#post3413886

It's ugly but sounds great.

Click the image to open in full size.

With tiny 0.5 in copper tube ring heatsink (good for 30 w/ch)

Click the image to open in full size.

I have since made a proper PCB version using a TPA3116D2 (good for 50 w/ch) doesn't sound better just capable of higher power.
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