Obsolete IC replacement  L165V Power Amplifier - Page 1
Forum section: electronics / beginners

author: Zielsziek
author_rank: Contributor
timestamp: June 01, 2025, 01:30:22 pm
content: Hello, I want to build this curve tracer from the January 1996 Elektor Magazine issue .The part in question is a L165V Power Amplifier, IC2 on the schematic . I found a NTE1288 as a possible substitute, but the local parts supplier, TME, only has one in stock and I am reluctant to go for it. I was unable to find other similar power Amps.Thanks!

author: wraper
author_rank: Unknown
timestamp: June 01, 2025, 01:49:24 pm
content: Most likely you can just drop in TDA2030 (ST part is obsolete but there are plenty from Chinese manufacturers at LCSC), I don't see any obstacles on a quick glance. TDA2050 should work too.EDIT: looking into datasheets a bit more, TDA2030 has the same gain/frequency response curve, the same input stage. They look to be pretty much the same thing just marketed for different purposes.

author: nonius_
author_rank: Contributor
timestamp: June 01, 2025, 03:41:29 pm
content: Funny, I've been intending to build this circuit as well, and on a local Dutch electronics board they're now building this as a group-project (someone had PCB's made in China). The L165 was still easily locally available, despite being obsolete. (https://www.circuitsonline.net/forum/view/168516/lastIt's also possible to use a discrete amplifier: I have an Elektuur XR2206 function generator that was published in 2 different versions: one with a discrete power amplifier, another with a L165 power opamp. The latter circuit was the more modern one, easier to build, but at the cost of a specialized IC. See schematics in attachments below. Ignore the XR2206-part, just focus on the output amplifiers. Based on this, it should be pretty easy to replace the L165 with a discrete power amplifier. But, as I said, the L165 was still easily available over here. So ordering an L165 should also be an option.IIRC, there were other pin-compatible replacements for the L165 power opamp, but these were also obsolete and more difficult to obtain.Did you manage to obtain a BF256A? That was probably the hardest component to obtain, based on remarks in that Dutch thread. There's some good information in that thread, with input from some very experienced analog designers; I'd recommend machine-translating and reading it.

author: nonius_
author_rank: Contributor
timestamp: June 01, 2025, 04:01:53 pm
content: BTW, about half a year ago I built my own curve-tracer, based on a design by Elliott Sound Projects (#119, not #106 as is erroneously indicated in the picture, with a few extra features to make it more universal and useful to me. The only thing I regret is not building a curve trace 25-30 years ago. It has been a very worthwhile addition to the hobby lab.https://sound-au.com/project119.htmI added a voltage calibrator (8V and 40V) to more accurately measure voltages on the scope screen; negative voltage limiter, voltage limiters, dual channels (A/B) with either manual or automatic switching, an adjustable voltage source and a pulse-function with adjustable frequency, to test semiconductors (FETs) and relays.Test voltages are either 8.5V or 45V; the latter is also handy for testing white LEDs (with 8 or 10 diodes in series per package); easy to find a broken LED that way. Anyway, as I said in the previous post, I love my curve tracer! Also very handy for testing zener diodes and determining their voltage.

author: floobydust
author_rank: Super Contributor
timestamp: June 01, 2025, 05:05:17 pm
content: The L165 is unity-gain stable (edit: no, the datasheet circuit is misleading with a div 10 resistor, so it's 10 min.) but the common substitutes are not and oscillate at lower gains. I think this is something to consider.TDA2050 min. is Av=16 (24dB).TDA2030min. is Av=16 (24dB).LM1875min. is Av=10.LM675min. Av=10. (datasheet shows special comp? for unity).

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2025, 05:43:32 pm
content: Quote from: floobydust on June 01, 2025, 05:05:17 pmThat was the first thing I checked; audio power amplifiers are often designed with a minimum stable gain because decompensation increases gain-bandwidth product and slew rate.A resistor can be added across the inputs to increase the noise gain, allowing an LM675 or LM1875 to be used. Or a more exotic part like an OPA547 could be used. A composite amplifier with a couple of added discrete transistors would also work.

author: wraper
author_rank: Unknown
timestamp: June 01, 2025, 05:49:37 pm
content: For this design it looks like minimum gain requirement should be met with all of those, especially for >10, unless P1 is adjusted to minimum resistance.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2025, 05:50:27 pm
content: The circuit as shown has the resistor across the inputs and this way has quite some noise gain. So the TDA2030 is likely OK here.The discrete amplifier would be a reasonable alternative.The BF256A is not that critical. One would want a relatively small JFET with comparable thereshold. The threshold is anyway scattering quite a lot - so the amplitude could vary with the individual JFETS and capacitor / resistor tolerance. One may want some adjustment. I would consider somerhing like MMBFJ202 - many JFETs would work, maybe with minor adjustments.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2025, 06:40:35 pm
content: Quote from: Kleinstein on June 01, 2025, 05:50:27 pmThe L165V must also have a minimum stable gain.

author: wraper
author_rank: Unknown
timestamp: June 01, 2025, 07:22:56 pm
content: Quote from: David Hess on June 01, 2025, 06:40:35 pmActually yes, it's >10 as well in normal connection. Unity gain configuration shown in datasheet is fake unity gain as it's divide + amplify.

author: floobydust
author_rank: Super Contributor
timestamp: June 01, 2025, 07:34:05 pm
content: Sorry I missed that L165 unity gain circuit is fakeIt's just that I had tried to get a TDA2030's voltage gain down, finding it's too much for an audio power amp, and I never succeeded. It just oscillated.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 02, 2025, 02:23:16 am
content: The fake schematic has the same “look” of legitimate STMicro data sheet drawings.Where did you copy it from?

author: floobydust
author_rank: Super Contributor
timestamp: June 02, 2025, 02:38:28 am
content: I took the snippet from theL165 datasheetI call it fake because as was pointed out, it's unity-gain yet operating at a gain of 10.

author: Zielsziek
author_rank: Contributor
timestamp: June 02, 2025, 06:12:50 am
content: Thank you all for the valuable insight. I do have a BF256A, the only part that had me scratching my head was the L165 and the "unity gain". I will definitely read up the Dutch forum as it seems to be packed with useful information.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2025, 12:15:18 pm
content: Quote from: floobydust on June 02, 2025, 02:38:28 amThe datasheet does not explicitly state that is decompensated for a gain of 10 or greater, but relies on the designer's experience to identify this.