John Curl's Blowtorch preamplifier

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#13,823
audiowolf said:
http://www.forsselltech.com/downloads/schematics/JFETMP1.PDF

Not bad sounding at all, but not absolute state of the art. They grind off the tops of all the plastic DIPs
Woof!
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That "Cohen" input stage dates at least from 1968, it was lifted from early 60's geo-phone amplifiers (pre JC!). Fuming nitric acid will remove the plastic and not damage the IC's at all, in fact they still work if the leads don't fall off. Not recommended for DIY.
 
#13,824
andy_c said:


I don' t think this would be the same thing. Let's say you had 10 relays each for the series and shunt arms (that is, the series and shunt arms would each have 10 resistors). This would give 1024 attenuation states. I'm not sure how you'd do this with a matrix, but I'm not familiar with the relay matrix idea either, so I may be misunderstanding you.
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Andy

To get 1024 steps with a matrix you would need a total of 10 relays ( 2^10)

Stinius
 
#13,825
Since we are talking about a preamp here, let’s assume that we are discussing a stepped attenuator that have a limited number of steps.

So my question is:
How many steps are needed?

Bonsai says 24.

And how would you use the steps from 0dB to minus ? dB?

Stinius
 
#13,828
andy_c said:
Here's a thought I had a while back for a "pot emulator" consisting of resistors and relays. The basic structure would be that of a series arm Rse and a shunt arm Rsh as shown in the picture below. Each of Rse and Rsh would consist of a bunch of series resistors with a relay across each one that is either open or shorts out its corresponding resistor.

Rse would look like this:

Rse = R + 2R + 4R + ... + 2(N - 1)R

Rsh would be the same as well, namely:

Rsh = R + 2R + 4R + ... + 2(N - 1)R

Suppose there were a digital word Xse controlling Rse such that the LSB of Xse opens or shorts R, the next bit opens or shorts 2R, and so on. There could be a second digital word Xsh that does the same for the individual resistor elements of Rsh.

Now, if Xsh were made to be the bitwise NOT of Xse, the input resistance would be constant, independent of the attenuation. So it would emulate a pot in that respect. The signal would go through a lot of relay contacts with this arrangement though. Has anybody else thought of this or tried it out?
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I've built an attenuator like that: http://www.diyaudio.com/forums/showthread.php?postid=1118699#post1118699
This is also what Placette RVC is doing.

Schematic in attachment, link to steps calculator: http://www.diyaudio.com/forums/showthread.php?postid=840215#post840215
 

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#13,829
This still uses a switch to give the steps.
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So? What's not going to involve a contact of some sort?

Not much heat generated by average preamp signal levels.
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You didn't get my drift. I'm not talking about warming your room here. Attenuation through resistance is changing the signal to heat and dissipating its power. Transformers maintain the power.


There shouldn't be any DC to block.
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Ideally yes, and transformers shouldn't have to deal with much. Would you rather block with a capacitor?


Any type volume control can be used balanced or unbalanced.
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For multiple mixed inputs, without extra switch contacts? What else can do all these functions? Some of the other suggestions here cost just as much.
 
#13,830
#13,831
Yes, Placette is very well regarded and I've been using it for a while myself. However, after initial excitement, I was starting noticing defficiencies, mainly the coloration.

To me, TVC is still preferred way to adjust volume (based on S&B TX102)

Steve Dunlap said:
Any type volume control can be used balanced or unbalanced.
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Yes, it can. But most volume controls by itself will not do unbalanced to balanced conversion (and reverse), also aren't able to support polarity switching of unbalanced signal: TVC will do it without need for additional active circuitry.

But in the end, TVC is not perfect either. I have a chance listening without any attenuator now as the amp has just the right gain for normal levels and it's very revealing experience.
 

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#13,835
Just ni-----. Now, don't take me wrong, I joked about the same thing with the Blowtorch and our huge knobs! In fact, we once originally had even bigger knobs, (that I nicknamed Dolly knobs) but they were too much. It looked like the preamp would tip over because of them.:nod:
 
#13,836
john curl said:
No, it is just that the hear-no-difference group have nothing to contribute on the subject and dilute the topic with vague insults.:Pinoc:
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Actually it’s the inability of the GEB for the last 3 decades (or since whenever) to present credible evidence that they can actually detect most of the things they claim to. 😱 😱

Despite very real potentiometer/attenuator issues (frequency response limitation with rising impedance, noise, crackly pot wipers, oxidised relay/switch contacts, etc) claims of audible differences between ppb variations in resistor brands are about as credible as revelations regarding the second coming of Jesus.

Cheers,
Glen
 
#13,838
john curl said:
All we have to do, is that have 2 or more people hear the same quality in reproduction. Do you know who said that first? The late Richard Heyser. Perhaps, you should have known him, then you would be as blessed as many of us are. :wiz:
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LOL!

Now all we need is two or more people to hear the same voice in their head that the saviour will come down on Tuesday.

BTW unlike you blessed ones, I'm irrevocably damned anyway, so just in case it really happens, can I have your Porsche afterwards?
 
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