Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier" - Page 3 - diyAudio
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Chip Amps Amplifiers based on integrated circuits

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Old 31st July 2013, 12:48 AM   #21
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Quote:
Originally Posted by petefrontiers View Post
Ok, A little research told me what is an impedance plot. Sorry for my not perfect understanding of english language. But I would like to know how do you measure it.
There are of course tool that do the job, but a reasonable accurate AC meter and a signal generator is needed. We are mostly concerned with below 100 Hertz. You can also get CDs with test tones. Use a 4R to 10R resistor, adjust level to 1V RMS and take readings below 100 Hertz, and provided that it stays reasonable close to 1V RMS below 100 Hertz at different frequencies, then all is OK.

(For others reading this, the Elsinores have a much flatter impedance below 100 Hertz than almost any other speaker around - if it was put together properly.)

If the meter is accurate to higher frequencies, then you can measure that too, but below 100 Hertz is the most important.

The signal generator can even be a smart phone with a suitable app for iPhone or Android.

There are some YouTube videos, check them out.

Cheers, Joe

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Last edited by Joe Rasmussen; 31st July 2013 at 12:51 AM.
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Old 31st July 2013, 02:03 AM   #22
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Thanks Joe, I have everything on hand to do this test. Will do it when I will be back from vacations.
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Old 31st July 2013, 06:47 AM   #23
irext is offline irext  Australia
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Hi Joe. I'm looking on with much interest. I see two PCB's in there. Are they specific for the LM3875? i.e. are they just breakouts of the I.C. pins? If so how can I obtain them. Ultimately I'd to a complete PCB layout with Protel but initially those would get it up and running. I'm already thinking of chassis layouts. Hmmm. (Still loving my Elsinore's)
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Old 31st July 2013, 11:56 AM   #24
Fenris is offline Fenris  United States
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Nice Joe. I did almost the exact same thing two years ago with either the LM3875 or 3886, I don't remember which. I did a P2P with an old Gainclone and rewired it as a standard V to I converter. My sense resistor was a bundle of ten 10 ohm resistors in parallel- 1 ohm made the calculations easier. What I found was that they are very speaker dependent - you have to use fullrange drivers and small ones can easily run out of excursion around the resonant frequency. They do sound different, I think the rising impedance with frequency puts more power into the high frequencies of a driver with a current amp, whereas a voltage amp puts in less. So there's a differing frequency response.

The amp sounded fine, but I retired it as an experiment. I just don't like what happens if a speaker wire is accidentally unplugged or there is a dodgy connection while the amp is on. Maybe for a hardwired subwoofer this could be interesting.
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Old 31st July 2013, 03:11 PM   #25
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Quote:
Originally Posted by Fenris View Post
What I found was that they are very speaker dependent - you have to use fullrange drivers and small ones can easily run out of excursion around the resonant frequency. They do sound different, I think the rising impedance with frequency puts more power into the high frequencies of a driver with a current amp, whereas a voltage amp puts in less. So there's a differing frequency response.
Take a look at this:

Click the image to open in full size.

This is the Elsinore Mk5 speakers, not a problem.

Click the image to open in full size.

Cheers, Joe
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Old 31st July 2013, 04:03 PM   #26
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Have you measured the output impedance of the amplifier? Are you shooting for a particular amplifier output impedance?
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Old 31st July 2013, 05:41 PM   #27
dhenryp is offline dhenryp  United States
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Any thoughts on using this, or something like it, to directly drive a ribbon (impedance ~ .1 ohm). What is the limiting factor in the amount of current it can drive?
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Old 31st July 2013, 05:58 PM   #28
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Any thoughts on using this, or something like it, to directly drive a ribbon (impedance ~ .1 ohm). What is the limiting factor in the amount of current it can drive?
Nelson Pass talks on his DIY web site about a high current transconductance amp that he tested on tweeters.

The current limit will be a function of safe operating area of the chip and emitter resistors in the final stage of the chip.

My gut tells me that the lowest voltage rails that allow operation will be the best since your output voltage will be just a couple of volts if even that.

Your tweeter will look like a short circuit to the amp. Your cables may have more DC resistance than the tweeter.
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Old 31st July 2013, 06:09 PM   #29
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Quote:
Originally Posted by dhenryp View Post
Any thoughts on using this, or something like it, to directly drive a ribbon (impedance ~ .1 ohm). What is the limiting factor in the amount of current it can drive?
I would highly not recommend driving a ribbon with a transconductance amplifier. Tweeters and especially ribbons are voltage devices requiring very little current to drive them. Similar question was posed to me eons ago (~25 years or so) and devised a test to prove my point. It was winter in Ohio and as we all know static can be an issue if the humidity level drops too low. So I took a woofer and a tweeter and earth grounded the negative terminal, no crossover, just raw drivers. Then shuffled across the floor in my silk socks and touched the unconnected terminal of the woofer. Nothing, except the spark jumping the gap. Now for the tweeter, a very sharp crack could be heard. Since static electricity has ultra low current, the woofer couldn't be excited, but on the flipside the tweeter reacted as predicted, proving the point.
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Old 31st July 2013, 10:24 PM   #30
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Originally Posted by Greebster View Post
I would highly not recommend driving a ribbon with a transconductance amplifier. Tweeters and especially ribbons are voltage devices requiring very little current to drive them. Similar question was posed to me eons ago (~25 years or so) and devised a test to prove my point. It was winter in Ohio and as we all know static can be an issue if the humidity level drops too low. So I took a woofer and a tweeter and earth grounded the negative terminal, no crossover, just raw drivers. Then shuffled across the floor in my silk socks and touched the unconnected terminal of the woofer. Nothing, except the spark jumping the gap. Now for the tweeter, a very sharp crack could be heard. Since static electricity has ultra low current, the woofer couldn't be excited, but on the flipside the tweeter reacted as predicted, proving the point.
I believe that your experiment proved something about the mass of the ribbon compared to the mass of the woofer and did not prove anything about voltage or current.

A ribbon is driven just like a woofer; force is equal to BL time Current.

For a 0.1 ohm ribbon, 10 amps would be at 1 volt; V = I * R. I doubt very much that the ribbon discussed here can handle 10 amps.

I have built a 6 foot tall true ribbon and a 1.5 V battery caused the ribbon to jump out of the gap.

OP, if you can get 2 to 3 amps through your ribbon I believe that you will have more than enough. The limit of the current will be the chip, the wiring and the circuit design.
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