ImPasse Preamplifier

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#10
hifimaker said:
Where's the pictures or schematic being referring too?

-David
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Latest AudioExpress magazine. The picture anyways, some of it, in top right corner.
 

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#16
I want, I want

SY,

I was wondering just how I was going to drive the two Burning Amps in the F4 style when I get them finished. And then along came ImPasse. Thanks so much for this! Have in missed both BAFs I had no idea you had this cooking.

The article was really good and I'm psyched to build one of these. If circuit boards are in the offing I'm down.

If you have time I have a couple of questions about the design.

1. Can you elaborate on your choices in getting the signal from the voltage divider to the output stage? I'm unsure how you calculated the values for the voltage divider and it's overall effect on the signal going to the unity gain phase splitter.

2. I may want to add a balance control to the mix and would like to get some guidance on how to figure out the matching the volume/balance/R1/C1 combo with the prima donna input transformer. I don't have any transformers on hand so I'm pretty open.

My apologies if these are lame questions.

KB
 
#17
I'll have more time to answer in detail later, but here's the basics:

1. Voltage divider was chosen to get the right DC potentials on the ECC88 split load. The bypass cap on the first resistor turns it into a step network, but at a very low frequency so that the divider doesn't cause a gain loss in the audio band.

2. I'm not sure what's commercially available for balance controls, but I have some old Alps balance pots that are zero ohms through half of the rotation, then audio taper for the other half. That way, at midpoint there's no loss on either channel. I'd look for something like that, and use a value that's about the same as the volume pot- this will give you 6dB of adjustment which should be more than enough, and it won't perturb the input transformer.

Jackinnj is designing some boards which will drop into a PAS chassis. If he wants to do a run and sell them, that's fine with me. Otherwise, if someone else wants to get some quotes from a board house and get a group buy together, and it's OK with Jack, I'll certainly take a board myself.
 
#18
Thanks!

SY,

Thanks for the quick response. When you get time for more detail I'd be interested to know how you calculated the values, what components contribute, and such.

As for the balance pot, I wasn't clear on the type of pot you were describing and found an interesting set of pot definitions on www.rane.com. For those of you that didn't know about it either (hopefully, I won't be the only one!), here goes...

  • audio taper (aka A-taper): Usually 15% resistance at the 50% rotation point.
  • E-taper Similar to a reversed audio taper but with 25% resistance at the 50% rotation point.
  • linear taper(aka B-taper): Always 50% resistance at the 50% travel point.
  • log taper(aka D-taper): Often used as an audio taper since its 50% rotation point has 10% resistance.
  • MN taper (aka balance pot) Special taper developed for home stereo "Balance" controls. Consists of two sections (one for each channel) operating opposite each other. Exactly one-half of each section is a zero resistance surface (i.e., solid-copper or equivalent), the next 50% of travel is linear taper. Therefore for one channel rotating the slider through the first 50% of travel does not change the level at all, while the other channel is reduce from full to zero, and vice versa, with the middle position (usually featuring a center-detent) always passing full signal to each channel. See balance control.
  • RD-taper A reversed D-taper (see log taper above).

Could the Noble pot A-25 on this page be the item to which we're referring?

Thanks again. BTW, I'm not in any rush. Just excited about it!

K
 
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