Doug Self Preamp from Linear Audio #5

I have had quite a few inquiries for PCB kits of late. A few months ago I had let my supply lapse. Interest had fallen off and the cost to mail them overseas had skyrocketed. Cost to ship overseas reach $35 US.

My question is, "Is there enough interest to make another run?" It's a great project that cannot be beat for value or sound quality.

So, please let me know if there is a real interest.




I would like to get these РСВ.

Still in stock ?

I am located in Germany

Regards from Europe.
 
Of course you "can" run three inch wires to the pots. The real question is "will it degrade the performance of the circuit". The resistance of three inches of 22 gauge wire added to the resistance of the pots is not significant for these circuits, so no issue there. The potential issue would be the loop area the wires create. Loops are antennas, so they pick up, or more properly, "generate" a signal from fields that intersect them. The good news is that the gain of a loop antenna is directly related to its size. You can minimize the loop area of the wire loops you just created by braiding or twisting the wires together. This is the reason ground planes on circuit boards are so effective if properly implemented. They reduce the loop areas of signals and their return path. No magic foobydust, just physics.
 
Of course you "can" run three inch wires to the pots. The real question is "will it degrade the performance of the circuit". The resistance of three inches of 22 gauge wire added to the resistance of the pots is not significant for these circuits, so no issue there. The potential issue would be the loop area the wires create. Loops are antennas, so they pick up, or more properly, "generate" a signal from fields that intersect them. The good news is that the gain of a loop antenna is directly related to its size. You can minimize the loop area of the wire loops you just created by braiding or twisting the wires together. This is the reason ground planes on circuit boards are so effective if properly implemented. They reduce the loop areas of signals and their return path. No magic foobydust, just physics.
Thank you for the reply.
Radio frequency interference and instability are my prime concerns, and I didn't find any indicators of conditions that might introduce these problems. Long leads were common with older discrete designs, but they didn't always have the bandwidths found among op amps.
 
Can I run wires (probably 3 or so inches length) from the preamp board to the linear pots, so that the knobs can be arranged differently on the front panel of a preamp? Or is that asking for trouble?


I'd use shielded cables between pcb and pots, but that might no be necessary if the whole enclosure is metal and no high voltage wires are nearby. Run all the wires for one pot together to prevent loops as mentioned above.


3" is not that big unless the signal is tiny - at line level I'd not be very worried.
 
Thank you for the reply.

The OPA1692 and OPA1688 bipolars are pretty inexpensive, about the same cost as the LM4562. Noise is stated as being higher (particularly the 1688) and require adapters, but are there any reasons why these wouldn’t suit this preamp?

The OPA1688 is not a bipolar opamp, its current noise is 1.8fA/√Hz, 3 orders of magnitude lower than the LM4562.


The OPA1692 despite being bipolar still has much lower current noise than the LM4562.

So it all depends what the source impedance is as to whether either of these is noisier - people so often forget current noise.
 
Hey guys I just completed my build but its not working properly.
I'm not getting any sound out of the preamplifier.
I managed to locate the issue to the bottom BC550 on the IO board, when I put my multimeter on the Base and Collector sound is coming out but as soon as I disconnect it no sound again.
Can anyone help me and tell me how I'm supposed to repair this?
I already tried with two new BC550 to no avail.
Thank you
 
I managed to locate the issue to the bottom BC550 on the IO board, when I put my multimeter on the Base and Collector sound is coming out but as soon as I disconnect it no sound again.
Can anyone help me and tell me how I'm supposed to repair this?
The fault is not with that transistor, but the other BC550 or the RC network driving it.

The other BC550 is used as a 5V zener, which is a trick I wouldn't recommend, as this is not something guaranteed by the datasheet, but in theory it should work.

The free-wheel diode for the relay coil is missing, presumably because this circuit never switches off. I'd never omit this, its false economy - your testing with the multimeter will have switched the bottom BC550 on and off, thus risking frying it.

I would recommend replacing the other BC550 with a 4.7V zener, anode to the bottom BC550 base.

And I'd recommend a 1N4148 across the relay coils.
 
Hello Mark, thank you for answering so promptly.
While I'm very happy you answered I do not understand the totality of what you said unfortunately it's a bit beyond my knowledge.
I replaced the top BC550 already and did not change anything.
I already have a diode across each relay since I have the latest IO board.
I need to check if I have a Zener in stock.
 
Before doing that just measure the voltage across the capacitor - it should charge up in a few seconds from switch-on to about 5V and stick there - if it keeps rising toward 12V, the BC550 isn't doing the job and a zener is indicated.


If it never rises, suspect the resistor that's supposedly charging it. (and check the cap isn't shorted too)
 
It could be that modern BC550s are significantly different due to process changes, it could be that you have counterfeit BC550's even - but either way there's nothing in the datasheet that guarantee's the EB diode breakdown voltage, just that it isn't less than 5V (nearly all transistors have this same value in their data, its not actually a measured spec AFAICT.)
 
Hey Hicoco,

Here is a link to a google album with some pics, I don't know if it's gonna help: DOUG SELF PRE - Google Photos
The resistors and cap were properly soldered the first time I encountered the issue.
I'm now waiting for new cap and resistors to properly place everything.
To be honest I'm starting to doubt the authenticity of the BC550, but I don't see why they are running well on the power supply and not on the IO BOARD.
I bought them from an Italian shop on eBay that was well rated.
I bought some on Banzai Music in Germany today just in case I'd need some more.

Thank you for the help anyway.