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Old 28th May 2011, 04:36 AM   #721
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Quote:
Originally Posted by weinstro View Post
I saw somewhere in this thread that the closed loop gain is 29.5 dB. For 68W out at 4 ohms (rated output of LM3886 at +/- 28 Vdc), I get 0.55 Vrms input. Is that correct?
You can't safely get 68W out of a MyRef for several reasons:

1) At +/- 32V rails (close to optimal for sonics), the LM3886 is not spec'ed to drive 4 ohm loads, only 8 ohm loads. You could however try 6 ohms at +/- 28V and hope that SPike doesn't get triggered.

2) There's a series resistance of 0.47 ohm in the Howland current pump that scales the output voltage swing into the load by the ratio RL/(0.47 + RL). This reduces the output power by the square of that ratio, making it progressively lower for lower RL, i.e. worse for 4 ohm loads than 8 ohm loads.

3) It's more stable at higher RL, both due to SPike as well as the phase margin from the Rev C compensation schema, which is optimized for 8 ohms.

You can reasonably expect ~40W max into 8 ohms with ~25V amplitude at the output at +/- 32V rails.
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Old 28th May 2011, 11:21 AM   #722
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Does this reasoning mean you shouldn't use the Rev C to power 4 ohms speakers, even if at lower supply voltages?

Isn't that a huge limitation for a power amplifier?
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Old 28th May 2011, 12:20 PM   #723
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Originally Posted by carlmart View Post
Does this reasoning mean you shouldn't use the Rev C to power 4 ohms speakers, even if at lower supply voltages?
No, just use a 2x22V trasformer and it runs fine at 4 Ohm (and 8 Ohmm too).

It's just that if you have 8Ohm speakers you can go a bit up with voltage (2x25V) to have a bit more power.
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Old 28th May 2011, 01:17 PM   #724
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To talk about "a bit more power" is relative. What's the difference between 40 watts @ 8 ohms and 40 watts @ 4 ohms?

What I wonder is how dynamics are affected with heavier loads, like 4, 3 or 2 ohms, which is what may happen with real world loads.

That is: an 8 ohm speaker might dip lower in impedance, particularly at low frequencies, like 5 or 6 ohms.
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Old 28th May 2011, 01:38 PM   #725
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Originally Posted by carlmart View Post
What I wonder is how dynamics are affected with heavier loads, like 4, 3 or 2 ohms, which is what may happen with real world loads.

That is: an 8 ohm speaker might dip lower in impedance, particularly at low frequencies, like 5 or 6 ohms.
Mauro Penasa specced this amp at 22V for 4Ohm loads and at 25V for 8Ohm loads.

Loads intended as speaker manifacturers intend, dips are normal and the MyRef is perfectly able to handle them.

Quite all people built this amp is happy and from what I know the MyRef drives a variety of speakers with authority.

Obviously I don't thinks it's the best amp for an electrostatic speaker set...
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Old 28th May 2011, 02:10 PM   #726
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Quote:
Originally Posted by weinstro View Post
For the myRefC, how much input voltage is required to reach rated output power?
Assuming 25V output amplitude for the rated max power, the feedback divider network of 12k and 390R gives ~786 mV amplitude at the opamp input. There's an input attenuator of with a ratio of 1/1.033, so that gives ~812 mV at the input connector block. Divide by ~1.4, giving ~575 mVrms - almost the same as your estimate (which may not have taken the input attenuator into account).
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Old 28th May 2011, 02:44 PM   #727
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Originally Posted by ClaveFremen View Post
Mauro Penasa specced this amp at 22V for 4Ohm loads and at 25V for 8Ohm loads.

Loads intended as speaker manifacturers intend, dips are normal and the MyRef is perfectly able to handle them.

Quite all people built this amp is happy and from what I know the MyRef drives a variety of speakers with authority.

Obviously I don't thinks it's the best amp for an electrostatic speaker set...
No static speakers for it, unfortunately. But I am working on a nearfield speaker project with 4 ohm drivers, so I was wondering if the MyRefC was up to the job. I was certain it was, and I was surprised at the output swing limits mentioned that I might have.

Let's see: 22v + 22v should be around +/- 29VDC, right? Can I expect around 55w @ 4 ohms? Of course I would never listen at that continuous power, but it's good to know I have some 6 or 9dB to swing up from 10 watts, which is a reasonable level for nearfield.
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Old 28th May 2011, 04:15 PM   #728
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Originally Posted by linuxguru View Post
1) No buffer is needed, the on-board LM318 stage provides sufficiently high input impedance (~100k). However, if you use a passive attenuator before the MyRef, then you may need a buffer, depending on your source impedance.

Well, I was revising the buffer question I had asked and found a bit of an incongruence there.

Of course I am asking if I would need a buffer for using a passive attenuator, a stepped or simple volume pot, even a balance control. What about an LDR attenuator? Can I connect it directly?

I was considering something simpler, like a 21-step, 10K attenuator you can get on eBay. But I wouldn't want to use a buffer, if possible.
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Old 28th May 2011, 04:34 PM   #729
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Just search on eBay "DACT Type 21 Stepped Attenuator"... 10K value.

This little thing sounds incredibly good.

I'm still trying to make my DCB1 + LightSpeed attenuator sound at least as that attenuator alone!
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Last edited by ClaveFremen; 28th May 2011 at 04:37 PM.
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Old 28th May 2011, 04:42 PM   #730
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Originally Posted by ClaveFremen View Post
Just search on eBay "DACT Type 21 Stepped Attenuator"... 10K value.

This little thing sounds incredibly good.
That's the "little thing" I was considering using for this project. It seems neat and very affordable. But I was concerned if I needed a buffer.

Quote:
I'm still trying to make my DCB1 + LightSpeed attenuator sound at least as that attenuator alone!
What's a DCB1?
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