Au contraire. I have relatively poor hearing. I'm just trying to figure out how to use the gear I have without destroying anything!
You can choose the type of input independently from the type of output. So don't worry and just enjoy listening 🙂
Ah. You answered the question I should have asked: Does the type of input have to match the type of output?
Question regarding the output stage
After a long time of enjoying this preamp, I come back with a design question:
I had a look at the output stages in the "Small Signal Audio Design" book and I was wondering if it would improve the performance if we moved the 68R output resistors WITHIN the feedback loops of the output opamps (s. "zero impedance outputs" in the book).
Can someone maybe give some explanation on this?
Thanks a lot!
After a long time of enjoying this preamp, I come back with a design question:
I had a look at the output stages in the "Small Signal Audio Design" book and I was wondering if it would improve the performance if we moved the 68R output resistors WITHIN the feedback loops of the output opamps (s. "zero impedance outputs" in the book).
Can someone maybe give some explanation on this?
Thanks a lot!
The output resistor is "not seen" because it is within the feedback loop. However, these circuits tend to be unstable.
> if it would improve the performance if we moved the 68R output resistors WITHIN the feedback loops
Why? For what?
In domestic audio lines, 68 ohms is "nothing" relative to tens-of-K-ohms and tens-of-thousand-pFd.
In "long" (miles) audio lines we get echos which are tamed with termination. Double-terminated for transcontinental (and much radio), but across a city it is sufficient to put 50-100 ohms on the sending end so the line echos just once (not at the destination).
All amps are low-pass amps so all NFB amps get unstable loaded in naked capacitance, as the impedance goes to *zero*. A series resistor not-small compared to opamp effective impedance (so, 50-200 Ohms) breaks the zero.
If you are really up against a wall, as Jensen was thinking on his 990 opamp, you can wind a dozen turns on a 47 ohm resistor outside the NFB loop. The impedance will be near-zero to above the audio band, and then increase to 47 Ohms.
Why? For what?
In domestic audio lines, 68 ohms is "nothing" relative to tens-of-K-ohms and tens-of-thousand-pFd.
In "long" (miles) audio lines we get echos which are tamed with termination. Double-terminated for transcontinental (and much radio), but across a city it is sufficient to put 50-100 ohms on the sending end so the line echos just once (not at the destination).
All amps are low-pass amps so all NFB amps get unstable loaded in naked capacitance, as the impedance goes to *zero*. A series resistor not-small compared to opamp effective impedance (so, 50-200 Ohms) breaks the zero.
If you are really up against a wall, as Jensen was thinking on his 990 opamp, you can wind a dozen turns on a 47 ohm resistor outside the NFB loop. The impedance will be near-zero to above the audio band, and then increase to 47 Ohms.
@diyralf
@PRR
A big thank you to both of you for the short and long versions of the answer!
🙂
Had pretty much fun reading the illustrative long version 😉
@PRR
A big thank you to both of you for the short and long versions of the answer!
🙂Had pretty much fun reading the illustrative long version 😉
I had a Doug Self preamp for the 70's bnuilt by my fatjer I am trying to rebuild. Interested to know if this kits still available. If so, would like to purchase.
Designed back power entry for a filtered/switched cable entry module but now have forgot which one. My opening is 29mm x 49mm. Anybody have any ideas on that one?
There are backpanel files available in this thread where you can check this.
So Putting all this together into 1U preamp case from DIY store. Having some issues with the folded lip of the top and bottom. With the pot holes centered on the long dimension halves places the pot solder traces right on the bottom lip. It looks to be an interference to me. I'm grinding away the lower lip on mine to give adequate spacing. Anyone else see this also? I also am off-boarding my volume pot and mounting to faceplate to give a more balanced look on faceplate and allow much bigger volume knob. If anyone is interested I can post front and back FPD.files. They are mostly the same as Carls but with some mods. Paid homage to PS Audio PS IV preamp with some lettering in light blue. Always liked that look.
Off-boarding the volume control seemed absolutely a must to me. Can't imagine a volume control without remote control, so a separate board for it is a must. Want to use a motor pot.
Jims_audio on eBay has a great 7-bit stepped attenuator volume control which is driven by a simple ADC which takes its input from a voltage. So you can use a mono pot, feed its input to his stepped attenuator pot and get a great volume control. Remote control or manual, your wish. The actual rotary pot never carries the audio signal.
Jims_audio on eBay has a great 7-bit stepped attenuator volume control which is driven by a simple ADC which takes its input from a voltage. So you can use a mono pot, feed its input to his stepped attenuator pot and get a great volume control. Remote control or manual, your wish. The actual rotary pot never carries the audio signal.
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The Signal Transfer Company
I was looking at the website yesterday. The Signal Transfer Company: Precision Preamplifier
I was looking at the website yesterday. The Signal Transfer Company: Precision Preamplifier
Just a warning, they are notoriously difficult to order from. As discussed in this thread: The Signal Transfer Company
No, I have been keeping it in mind. Haven't built the preamp yet. Jims_audio will also sell you the remote controlled motor pot kit for it.
Hi
I just wonder if there is some print layout or more detail than The scemstic That Come with The Bom.
I never recived any documetation when i recived The boards.
Just now i have one channal that has half The gain compare to The other.
And tone controll its makes feedback sounds.. (Like a mic close to The loudspeaker) So something its really wrong.
I tried to resolder everything.
But no luck yet.
So here comes The importance of scjematics.
To trouble shoot where its going wrong.
Thanks again for a great forum.
I just wonder if there is some print layout or more detail than The scemstic That Come with The Bom.
I never recived any documetation when i recived The boards.
Just now i have one channal that has half The gain compare to The other.
And tone controll its makes feedback sounds.. (Like a mic close to The loudspeaker) So something its really wrong.
I tried to resolder everything.
But no luck yet.
So here comes The importance of scjematics.
To trouble shoot where its going wrong.
Thanks again for a great forum.