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Chip Amps Amplifiers based on integrated circuits

Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
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Old 29th August 2017, 07:33 PM   #351
Sharkythefrog is offline Sharkythefrog  Sweden
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Originally Posted by Joe Rasmussen View Post
Arguably, the component value that makes the biggest difference is the 1n5 below, the input filter.
Any preferences of type also on this one?
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Old 29th August 2017, 11:17 PM   #352
Joe Rasmussen is offline Joe Rasmussen  Australia
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Any preferences of type also on this one?
I have a selection of values of high quality 1% polypropylene non-inductive film, 100p to 10n @ 50V. Top drawer, made by Philips way back and picked up 10,000 in three boxes for $50. Johnny on the spot and didn't haggle the price. Maybe they didn't realise the value, but I sure did. Just paid and kept a straight face and said I would take them off their hands and that they would come in useful from time to time. Deal done!

But I would think Wima FKP would be good, but there should be others similarly.

.
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Old 1st January 2018, 09:06 AM   #353
augustoriodosul is offline augustoriodosul  Brazil
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I wish you a happy new year for everyone. I have some doubts because I am a newbie in electronics. I address a few questions to Mr. Joe Rasmussen.

1 - The sink recommendation and direct current through 0R33 indicates being in front of an amplifier type class A? If this is true, would not the heat or overcurrent drive the SPiKe (SOA) protection chip?

2 - Its power indication is 35VDC but LM3575 runs from 10 to 42VDC. If you use 25 VDC for less power, is it possible?

3 - Apparently there is no negative feedback, how is your gain regulated? Is the 4K7 resistor in the input?

4 - Dual mono construction is expensive: can not even PSU with two outputs, two winding transformer, two bridge rectifiers and two electrolytic banks?

Thank you very much for the help. Augusto J. Hoffmann - Brazil
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Old 1st January 2018, 10:05 AM   #354
Joe Rasmussen is offline Joe Rasmussen  Australia
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Quote:
Originally Posted by augustoriodosul View Post
I wish you a happy new year for everyone. I have some doubts because I am a newbie in electronics. I address a few questions to Mr. Joe Rasmussen.

1 - The sink recommendation and direct current through 0R33 indicates being in front of an amplifier type class A? If this is true, would not the heat or overcurrent drive the SPiKe (SOA) protection chip?

2 - Its power indication is 35VDC but LM3575 runs from 10 to 42VDC. If you use 25 VDC for less power, is it possible?

3 - Apparently there is no negative feedback, how is your gain regulated? Is the 4K7 resistor in the input?

4 - Dual mono construction is expensive: can not even PSU with two outputs, two winding transformer, two bridge rectifiers and two electrolytic banks?

Thank you very much for the help. Augusto J. Hoffmann - Brazil
Hi Augusto

1. Not at all, and it is only slightly Class A.

2. Yes, should be possible with 25V, clearly somewhat lower power. But still needs a reasonable heatsink.

3. Yes, there is negative feedback. The gain is related to the value of the load. For that reason you will have twice the gain at 8 Ohm compared to 4 Ohm and 16 Ohm double the gain of 8 Ohm. That is because the output is current, so 1 Amp current will give 4V into 4 Ohm load, 8V into 8 Ohm and so on. Weird I know, but that is the nature of the beast.

4. As long as you have isolation, then separate windings should work, but you would need four secondary windings on the same transformer, not something you see often. Keeping everything separate and isolated reduces potential hum. The diagram in Post #1 will guarantee no hum, but I can't predict all other possible variables and they can be very hairy indeed.

Hope that helps, cheers, Joe.
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Last edited by Joe Rasmussen; 1st January 2018 at 10:09 AM.
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Old 1st January 2018, 06:50 PM   #355
planet10 is online now planet10  Canada
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Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
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Originally Posted by Joe Rasmussen View Post
2. Yes, should be possible with 25V.
+/-25V DC for the rails is the sweet spot for the LM3875 if you want to driver 4Ω speakers.

dave
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Old 1st January 2018, 09:16 PM   #356
Joe Rasmussen is offline Joe Rasmussen  Australia
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+/-25V DC for the rails is the sweet spot for the LM3875 if you want to driver 4Ω speakers.

dave
Hi Dave

Could well be, hadn't thought about that. Cheers, Joe.
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Old 1st January 2018, 09:37 PM   #357
planet10 is online now planet10  Canada
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Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
If you look at the datasheets with a 4Ω load anything above 25VDC becomes current limited so you don’t actually get more power. I have been using a 3875 (Peter Daniels) at this voltage for years (in my office system). I may get Daniel to convert it to a variable TransAmp.

dave
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Old 1st January 2018, 10:48 PM   #358
thiagomogi is offline thiagomogi  Brazil
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Hi Planet10.
Have you heard in your speakers a transconductance amplifier?
just curious about your opinion.
I have very good impressions with my current amplifiers and variable impedance

Thanks
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Old 1st January 2018, 11:36 PM   #359
planet10 is online now planet10  Canada
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Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
I have heard many speakers on both Daniel’s original TransAmp based on a rework on an amp he built to drive a CRT and the LM3875 one he reworked from the same boards as in mine. Bothe were very good… the 1st right up there with the best amps i have heard.

With the “TransAmp” dial you can set the output impedance to what best suits the speaker you are listening too.

dave
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Old 2nd January 2018, 07:38 PM   #360
Joe Rasmussen is offline Joe Rasmussen  Australia
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If you look at the datasheets with a 4Ω load anything above 25VDC becomes current limited... I may get Daniel to convert it to a variable TransAmp.

dave
Point well taken. Yes, I would convert the amp too. Cheers, Joe.
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