Help with diode ID?

[analogguru]

There's been a lot of confusion and speculation between this current-regulating diode and the other that's used to bias the capsule.
I'm out.

Maybe this could be a little bit helpful ?

(D2 is the current regulating diode)

 

[abbey road d enfer]

Maybe this could be a little bit helpful ?

(D2 is the current regulating diode)

Yes, although there is a missing dot at the connection between D1, Q2, C2 and R2.
Do you happen to know the xfmr ratio?

 

[k brown]

Different symbol for the top diode, just a different style symbol, or different type diode?

 

[analogguru]

Yes, although there is a missing dot at the connection between D1, Q2, C2 and R2.
Do you happen to know the xfmr ratio?

Yes, the dot is missing in this factory schematic.

I don't know about the PRO37R and I haven't seen any guts of an AT8533, but I think it will be a 1:1 center tapped transformer like in the AT2020 or AT8531 with a similar circuit.

 

[analogguru]

Different symbol for the top diode, just a different style symbol, or different type diode?

Only different style symbol.

 

[k brown]

Must be 1:1, as my transformless mod has the same output level.

 

[abbey road d enfer]

Must be 1:1, as my transformless mod has the same output level.

OK, thanks. What is the spec'd sensitivity for this mic?
Someone recently asked about the actual capsule sensitivity. Quite often the figure provided by the manufacturer is the output at the FET's drain, which is not the level at the gate of the FET.
With this circuit, we know the output is the same as the capsule itself.

 

[k brown]

8533 and 8533x were used with several different mics containing the 2SK660; this one is typical.
The 8533x is the same circuit but with a switch to select between a small film cap to create a 6dB/oct HiPass, or the larger electrolytic shown in the 8533.

 

[k brown]

Wow - thread's gone dead, and still only one guess on the original question.

 

[ccaudle]

Wow - thread's gone dead, and still only one guess on the original question.

The original question was answered, perhaps you missed it in all the side discussion. You can use a standard diode, but 1ss226 is a dual diode, so you will need 2 if you use standard through hole diodes. Connect them in series so that the forward voltage drop is doubled.

 

[k brown]

Khron opined that a 4148 should work; that was the one guess I alluded to - didn't miss it. No other conclusions were offered, and nobody else confirmed his suggestion.

The 1SS226 was only mentioned as a clue to what the through-hole part in the 8533 might be - it's used in circuits similar to the 8533, but not the 8533, which has a through-hole glass diode.

 

[murrayatuptown]

Maybe I'm not adding anything useful with this remark, but it is common in vibration testing to use current sources to power JFET-based accelerometer signal circuits. I am not sure (forgot) about op-amp-based ones...I looked at many all at once to teach myself a bit to work on an accelerometer signal conditioner. The signal transducers in both cases (condenser and accelerometer) are both very high-Z. For accelerometer applications, I think one advantage of current source power is insensitivity to cable length. That probably is not the reason here. I think the other is noise performance.

The 'common' diode looks to me like it essentially clamps the JFET power source at 'a diode drop'...but at low current that is less than the textbook 0.6-0.7 V (Si). It would also protect the JFET from the variable voltage resulting from the current regulator compliance (the range of voltage over which the current regulator maintains it's current).

I have no idea how well JFET's work at such low voltages, but bought some tiny JFET's made for ECM capsules. They were both '20 V' parts but only provided curves up to 5V...I had other plans.

 

[ccaudle]

- it's used in circuits similar to the 8533, but not the 8533, which has a through-hole glass diode.

I apologize, I think I read too quickly and misunderstood your description.

If it is not clear whether the through hole diode is a single or a dual, you should be able to tell by measuring the voltage across the device. With a capacitor across the device a lot of the differences between different types of diodes are going to be swamped by the capacitor, so I would use whatever you can get. Diode packages tend to be standardized, so if you eventually find out the original device used by AT and want a more exact match you should be able to drop in a different diode to the same footprint.

 

[k brown]

Thanks for that - I'll start with a 4148 and see if it behaves like the factory example.

Would also love to get some opinions on the role of tantalums by AT in this circuit, especially the DC block on the input to the transistor, as seen in the schem in my first post (arrow in photo). Clearly the choice was not made for size, as the electro right next to it is the same value and volt rating (10uf/6.3v).

Lots of conflicting info online as to tant vs electro in audio signal path; the consenus seemed to be that when these were made ('80s) tants had certain advantages over electos in this role, but that that's no longer true of current electros. The cap is so 'cruising' in the circuit, that none of the usual tantalum failure modes would seem to be of concern.

Any thoughts? If you were doing a scratch-build clone why would you choose one over the other?