How Closely Do The Values Of Capacitors From The Same Lot Match?

[mediatechnology]

If you look at the old Transamp mic preamp application notes, they included CMRR trim pots. IIRC there were two trims a HF and a LF trim.

FWIW I never used trims or selected parts in manufacturing.

JR

The Transamp did have separate trims: https://www.ka-electronics.com/images/p ... ations.pdf

[Audio1Man]

The THD% in resistors that are shown in the table maybe misleading. What frequency & power levels did he use in testing the THD?

During the development of the ATS2 (the first all SMD AP) I had 0.0006 THD% until the 10Hz test when it went sky high. At first I thought it was oscillating. Probing around, I saw none. When I started touching around the THD went down. The DAC current to voltage stage feedback resistor was 0805 25ppm resistor was running about 45mw. The thermal mass in SMD is so small it heated and cooled cycle by cycle. Trying RN55 resistor and all was good. I did use SMD however the circuit power was reduced. Problem resolved.

Duke

[mediatechnology]

The test circuit, values, frequency and levels are given here and also in the original thread: http://www.waynekirkwood.com/images/pdf ... nesong.pdf

Thermal mass is also an issue with SMT and small geometry transistors.

[JR.]

The THD% in resistors that are shown in the table maybe misleading. What frequency & power levels did he use in testing the THD?

During the development of the ATS2 (the first all SMD AP) I had 0.0006 THD% until the 10Hz test when it went sky high. At first I thought it was oscillating. Probing around, I saw none. When I started touching around the THD went down. The DAC current to voltage stage feedback resistor was 0805 25ppm resistor was running about 45mw. The thermal mass in SMD is so small it heated and cooled cycle by cycle. Trying RN55 resistor and all was good. I did use SMD however the circuit power was reduced. Problem resolved.

Duke

That sounds reminiscent of the known LF distortion mechanism associated with loudspeaker fuses....

JR

[mediatechnology]

The THD% in resistors that are shown in the table maybe misleading. What frequency & power levels did he use in testing the THD?

During the development of the ATS2 (the first all SMD AP) I had 0.0006 THD% until the 10Hz test when it went sky high. At first I thought it was oscillating. Probing around, I saw none. When I started touching around the THD went down. The DAC current to voltage stage feedback resistor was 0805 25ppm resistor was running about 45mw. The thermal mass in SMD is so small it heated and cooled cycle by cycle. Trying RN55 resistor and all was good. I did use SMD however the circuit power was reduced. Problem resolved.

Duke

That sounds reminiscent of the known LF distortion mechanism associated with loudspeaker fuses....

JR

I've also seen this with SMT mic preamp ICs where there was a void between the IC and the thermal pad.
The thermal component is on each half-cycle which modulates the signal.
It manifests at low frequencies as third harmonic distortion.
The thermal mass of the IC body acts like a low-pass filter so the effect reduces as frequency increases.

I've seen an article somewhere or maybe it was Bcarso regarding the older CA-series transistor arrays.
Intersil or Harris IIRC have a pin-compatible high fT replacement for one of the arrays which, owing to being small geometry, have lower thermal mass than the original.
The discussion was due to the small geometry SMT replacement part having higher distortion.

[JR.]

This is a known issue to IC designers as they struggle to manage heat flows inside and away from IC substrates. Not only distortion but expected device junction voltage changes with temperature shifting operating points.

I used lots of cheap transistor arrays and they have their share of quirks, I never encountered obvious heat related issues.(More like I ignored them, I'm sure there were temperature related errors in logging applications).

JR

[mediatechnology]

I got a shipment of Wima film caps in today from Mouser and thought it might be fun to measure the values and see how closely they match.

I received 12 Wima MKP1F041005100JSSD.
These are a large 1 µF/250V/5% polypropylene units.
I normally use the 100V version but they were on back-order and the 250V are only a few mm taller.

I used 1 kHz as the test frequency.
The first unit measured 1019 nF and it became the "0%" reference.

Of the 12 units only one had about 3% error; the remaining 11 were <2%.
One unit matched almost perfectly, <0.1% to the test unit.
I set those two aside to keep them matched.