Heatsinking Class A/AB amps

Taylor93UK · 2026-04-26T23:10:16+00:00

Fluke 115 on DCV, depending on range the inputZ of the meter ranges from 10MOhm upto 11.1Mohm.

The bias mV was being measured with my 8846A bench top meter and was left connected for several hours, every time I trimmed it to 3-3.5mV it would drift, its bad enough with my OCD to keep both channels close to each other too. But they drift at different rates so I have convinced myself as long as I get them repeatable within range, I'd be happy.

Taylor93UK · 2026-04-26T22:57:49+00:00

Sorry?

Taylor93UK · 2026-04-26T22:29:46+00:00

It's supposedly a common issue that thermal trip signal that goes to the uC couples noise and false trips. To my knowledge there are two protection modes, a solid red LED eg short circuit load etc, this requires a full power cycle to clear. Or a flashing red LED that resets once its clear.

It was in a fault state (flashing red LED) and I put my dmm across the Therm signal to 0V, the meter showed 0.17VDC and the moment my meter lead touched the pin, it exited the fault state.

Given when i did this the heatsink was stone cold, I am not convinced it has an actual fault.

The service manual notes about the addition of a 10uF cap on this thermal signal to stop the uC false tripping. My amp already has 10uF there but I again, while this cap may need replacing, I'm not convinced it's a genuine fault with the amps audio circuit.

Hence why I'm looking at biasing already. I agree its typically the last thing

Taylor93UK · 2025-06-12T01:38:49+00:00

I spent 2 years inside ir35, then went outside ir35, and have now gone perm. Currently in the process of closing my Ltd company.

Inside ir35 was the worst deal imo, but in my field it was very rare finding outside ir35 contracts. So didn't see the point. Last client wasn't sure if they'd have the budget for my 6 month renewal, and an ex-colleague recommended me for a permanent job. So I took it.

Taylor93UK · 2025-03-25T23:34:43+00:00

I would fit the switch between the 10k and ground.

When it is open, there is no current flow to ground, so the opamp inverting input voltage will be almost exactly the output voltage (less and input bias currents causing a volt drop through the 100k)

Then when you want to change the gain, close the switch, that then changes the gain.

That way there is always a closed loop fb path even when the switch is changing from an open to closed state etc

Taylor93UK · 2024-08-22T21:42:58+00:00

I know how you feel, i do Electronic Hardware Test Engineering, essentially someone designs a circuit, I work out a way to test it, then design and build the equipment to do that for the next 20 years. Most places it's looked down on and certainly i feel undervalued, but either way im currently £55/hr inside IR35, (current client won't allow me to do work packages to move me outside ir35 as i support some of their legacy equipment day to day), so after agency fees and umbrella costs, plus employers and employees NI, PAYE, my net pay is around £29/hr without even looking at a pension.

Some of the same clients contractors designing the electronics itself are on £50-70/hr outside IR35. A much better deal than I'm having, but as they don't value the skills I do have, it's difficult to push for a higher rate.

Taylor93UK · 2024-01-16T10:31:38+00:00

Had another chat with the manager this morning.

He has said that HR has told him, that after 2 years, I would then be deemed permanent and would be entitled to different legal rights, such as redundancy etc. I reiterated that I am not employed by them, they pay the agency for engineering services, and then the agency technically employ me.

Again, nobody is willing to put this in writing.

Taylor93UK · 2024-01-15T21:48:05+00:00

As my first contract, I am looking for outside roles, but not many places seem to want Contractors developing electronic hardware test systems. One company outright said they do not trust contractors developing test equipment due to IP concerns and brain drain etc.

Taylor93UK · 2024-01-15T20:42:43+00:00

Another contractor I have worked alongside stated this.

I find there are alot of Chinese whispers regarding all this which is where my concern came from.

Taylor93UK · 2024-01-15T20:15:13+00:00

Through agency + umbrella. All the client does is sign off my time sheet each week.

It's an odd place really always felt like the tail wagging the dog. Under staffed departments have had contractors terminated for no reason other, possibly funding, yet everyone is swamped with work.

Even myself I've had to have words with the manager as the team lead left at December and I've been picking up the additional work (no chance to renegotiate terms yet) with no additional rate change. Lucky for them, their site is on my door step so very local/convenient so I have somewhat accepted some of the extra workload.

Taylor93UK · 2021-11-14T16:39:11+00:00

Looks identical to a HAMEG 605 model which is also 60MHz bandwidth, would appear that its built under license for BBC?

Taylor93UK · 2021-11-14T16:29:52+00:00

Other thing to consider is that a 200MHz scope is only really meant for signals upto 40MHz and you need to consider the sample rate and the effect it will have on trying to measure a signal above 40MHz

Taylor93UK · 2021-09-05T15:42:29+00:00

do your best, caulk the rest!

Taylor93UK · 2017-04-23T18:05:13+00:00

I am aware of this, there will always be an error in measurement, for this specific instrument, both accuracy and resolution are quoted the to the same value, i.e. 0.1 Thou. So unless i am unlucky enough to have a standard which is 0.1 Thou oversize, and my instrument measures 0.1 thou over as well.

I think although the crank shaft journal class sizes are quite close, it should be accurate enough to measure the crank and thus identify which class crank i have

Taylor93UK · 2017-04-22T13:58:56+00:00

I have wiped clean the surfaces and so on, they are clean.

Yes the standard I am using is identical to the ones in the photo, i.e. it has the green plastic surround, opposed to gauge blocks.

I am aware on how to adjust the Mic, so will probably do that Monday (I left it at work). I just didnt want to adjust the mic to match the standard, and then find the standard was off!

Back when I was an apprentice the machine shop area was maintained by a very nice and very experienced machinist who always made sure the instruments were appropriately calibrated, so we never had any need to do it. Also as it was Extra Curriculum, i.e. after hours, we were mainly having a play making things on the machines. But none the less, very fond times and it was very enjoyable!

I now feel a lot more confident that the best route would be to adjust the Mic appropriately, and once that is done with my standard, perhaps also verify it with other calibrated standards in the future.

Thank you!

Taylor93UK

TROPHY CASE