DIY Front End 2022

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#284
The circuit must have the input capacitors. I also know that properly-sized coupling capacitors have so little effect on the sonics that I don’t worry about it at all.

Reducing offset to an absolute minimum is an interesting engineering exercise. (Though it will vary quite a bit with temperature.) And worth messing around with.

That said, you still will need the output cap.
 
#288
Nelson Pass said:
As a concept you can DC couple the whole thing with bipolar supplies and a pot to trim the offset.
Click to expand...

Oof, it makes me nervous to imagine removing C2 in particular, when using balanced inputs such as from an XLR connector. As its name suggests, DC coupling guarantees that if there's DC (offset) on the input, there will be DC on the output. But removing C2 means the DC on the input is amplified by 10X and presented at the output. Oof. If there were a "DC servo" to remove output offset -- sure, no problem. Without a servo, you're at the mercy of whatever gear someone connects to the input. You can trim a pot to give zero offset at the output with the input shorted; but then when somebody connects a source that has DC offset, kaboom, 10X DC offset at the output. Oof.

For balanced inputs, I'd feel a LOT safer including C2 (and C1 for symmetry) rather than removing them.
 

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#296
Hello out there,

I have to apologize for my measurements (post #272). I was downloading new asio-drivers. I think I had problems there... 🤔
Today I made a new measurement on the Frontend2022: input voltage was 500mV rms (sinus / 1000Hz), 10kOhm resistor at the output, I measured something around 2 V AC / 1000Hz at the output (with a Fluke).
I didn't want to confuse anybody or wanted to query measurements of Mr. Pass! I only wanted to see what makes that circuit I am playing with...
In the pics is, what I got today:
Cheers
Dirk
 

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#298
bmdduck said:
@6L6, @william2001- thanks for your replies. My speakers aren't really demanding 4 ohm loads - I use a pair of Zu and a pair of Linn Ninkas.

I don’t play very loud. The F4 seems to be a great sounding amplifier but requires some gain up front. I think the gain on the new front end is up to 10db?

Some amplifiers don’t need more than a pot to control volume - maybe the F5 is in the category. Also wonder is this front end with the F5 is too much rain overall for my use.

Thanks again - bmdduck
Click to expand...
The ratio of a couple of resistors sets the gain of an op-amp, discrete or otherwise. The .pdf in post #1, just beneath the parts list, explains which resistors you can change to adjust the gain for your own needs.
 
#299
Mark Johnson said:
Oof, it makes me nervous ......
For balanced inputs, I'd feel a LOT safer including C2 (and C1 for symmetry) rather than removing them.
Click to expand...

Well, DC couple the whole thing means removing all C1, C2, C3, not ?
Just like any IC opamp. This is just a discrete opamp, as in the original article.

To enable DC couple and improve bias stability, I would :
a) Use J111s for Q3,4 and much larger resistors at R7,8. This will improve bias stability significantly by degeneration feedback.
b) Add a diode D1 and a resistor R9, where D1 should be thermally coupled to Q5 to track its Vbe drift.

(b) will actually improve intrinsic linearity and reduce distortion as a side benefit.
Trim DC offset with R8.

Again, only Spice. Not built.


Cheers,
Patrick
 

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