My Way Over-Engineered 8 Ohm Dummy Load! - Page 1
Forum section: electronics / projects

author: Smoky
author_rank: Frequent Contributor
timestamp: May 22, 2020, 01:25:07 am
content: During my last stereo repair project I needed an 8 Ohm dummy load for testing the output. Since I plan to work on tube amplifiers and vintage receivers in the 20-50 Watt output range, I bought two Riedon 50 Watt 8 Ohm chassis mount wirewounds.I come to find out that non-inductive resistors should be used for audio, but what I bought, arenotthe non-inductive type.Since I had already mounted the wirewound resistors, I connected them to my LC102 meter, and they measure 1.02 and 1.04 microhenries respectively.Would it be best to find a non-inductive replacement or will these Riedon resistors work Ok? They appear to but I'm not really sure what the added inductance affects.I did find these Caddock MP9100 non-inductive TO-247 resistors that I think might work since I already drilled and tapped several #6-32 threads into the base of this four pound Apple Computer heat sinkhttps://www.digikey.com/product-detail/en/caddock-electronics-inc/MP9100-8-00-1/MP9100-8-00F-ND/3013579I did continue with the dummy load project...

author: Smoky
author_rank: Frequent Contributor
timestamp: May 22, 2020, 01:28:05 am
content: I started with the base and a piece of aluminum angle to mount a few speaker terminals and the resistors...The terminals work well since the banana to BNC cables to the oscilloscope and the speaker wires can be mounted simultaneously...I added a 12VDC plug too so to power two small Sunon fansbtw, they're actually running in this picture!They have what Sunon calls "Vapo" bearings. The fans were mounted using small SS pin nails cut and polished and pressed into holes drilled in the aluminum:I made the small cable clamp from an aluminum gutter outlet:I found out that my old security camera wall-wart fits the plug too:Here's a shot through the fins. I do feel a good breeze on the opposite side:And since there is still plenty of space on the base of the heat sink, and I have an extra pair of speaker binding posts, I may add one 4 ohm 100 Watt Caddock Thick-film TO-247 resistor and hang it off of the back?Anyway, I haven't much time since I work construction this time of year, I wanted to fit this in and be ready for the Fall electronics projects!

author: oPossum
author_rank: Super Contributor
timestamp: May 22, 2020, 01:56:14 am
content: Quote from: Smoky on May 22, 2020, 01:25:07 amIt depends what your use case is. If you are going to be looking at the amp output on a 'scope to see clipping and estimate power then ordinary power resistors are adequate. If you are doing measurements with something like a AP or QA box then you would want as close to a pure resistive load as you can get.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 22, 2020, 02:30:27 am
content: Quote from: Smoky on May 22, 2020, 01:25:07 amVery pretty - you do very nice work!

author: richnormand
author_rank: Unknown
timestamp: May 22, 2020, 02:30:55 am
content: Nice workThis is what I have been using for years on power amps to up to 1000W.It is a 10 ohm load but you can make an 8, 6, 4, 2 ohm if you like using a jumper with alligator clips. All resistive old type.Fill the plate with DI water or oil as needed. Just plain air is good to 100W with fan. Price performance ratio....Real speakers will be resistive, inductive and phase reactive with enclosure air load and resonators and crossovers networks as viewed from the amp.Just walking in front of the enclosure will change parameters.All depend on what you are trying to measure with its internal feedback loop (including the wires to the speakers)

author: ahbushnell
author_rank: Frequent Contributor
timestamp: May 22, 2020, 05:08:45 pm
content: Quote from: Smoky on May 22, 2020, 01:25:07 am1 uH will not make a difference. There is that much inductance probably in speaker leads. at 10 kHz the reactive impedance is .0628 ohms for 1 uH. .12 ohms for 20 kHz.Nice construction job.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2020, 08:09:46 pm
content: Low value wire-wound resistors lack enough inductance to matter. In the larger scheme of things, realistic results require a load which simulates the reactance of a loudspeaker with considerable reactive components.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 22, 2020, 09:02:55 pm
content: Speakers are inductive loads so standard wirewound resistors will work just fine - in fact, it's sometimes necessary to add extra inductance to truly emulate a speaker.

author: David Hess
author_rank: Super Contributor
timestamp: May 23, 2020, 12:49:17 am
content: Quote from: NiHaoMike on May 22, 2020, 09:02:55 pmThe inductance of the low valued wirewound resistors required is insignificant compared to the inductance of the voice coil (about 1 mH) and inductance of the mechanical resonance (about 50 mH).

author: Smoky
author_rank: Frequent Contributor
timestamp: May 23, 2020, 01:40:43 am
content: If I was to describe in a few words my current and future world of electronics, it would be to repair old stereo equipment and to be able to accurately measure their performance.My #1 goal for tube amplifiers is to bring the noise floor at idle to the lowest level as possible measured at the speaker terminals. No signal at input and we're talking about a mono tube amplifier with no preamplifier attached.Should I be probing the speaker terminals with or without a dummy load to get the most accurate voltage measurement?I am trying to measure the noise that the circuitry within the amplifier is creating/adding.Thank you for all of the great advice and insight

author: bdunham7
author_rank: Super Contributor
timestamp: May 23, 2020, 03:04:18 am
content: You should never power up a transformer-output tube amplifier without a connected load at all. And as for measurements and adjustments, generally everything is done with a load connected. Your resistor bank should see regular use if that is what you are doing.

author: coppercone2
author_rank: Super Contributor
timestamp: May 23, 2020, 03:08:08 am
content: its not overengineered at all. its unfinishedmetal grill, fuse, chassis, handle, thermostatwood is not even polished so dont start lol'the only over engineered part is the sticker on the heat sink

author: Smoky
author_rank: Frequent Contributor
timestamp: May 23, 2020, 04:01:37 am
content: Up until now, I've been using a small Bose 101 speaker when taking measurements. I just measured the Bose speaker with my DMM and it reads 3.8 Ohms.From here on, I'll use the correct value of dummy load that matches the speaker terminals.I went ahead and ordered one of the Caddock MP9100 4 Ohm 100 Watt resistors to add the bottom of the heat sink to cover that value. I'll just measure one channel at a time with it.Heck, I'm thinking now, it might even be more wise since using the same dummy load would remove a variable and more accurately reveal any difference in the "gain" between two mono amps.

author: oPossum
author_rank: Super Contributor
timestamp: May 23, 2020, 04:17:33 am
content: The two 8 ohm resistors you have can be connected in parallel for 4 ohms.

author: Smoky
author_rank: Frequent Contributor
timestamp: May 23, 2020, 05:20:41 am
content: Quote from: oPossum on May 23, 2020, 04:17:33 amYou guys are worth an absolute fortune

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 23, 2020, 11:30:23 am
content: Don’t forget the leads have resistance, so that amp load will be a bit more than 8 ohm, 4 ohm. ie use thick leads.

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 23, 2020, 10:24:51 pm
content: Big ass dummy load thread? Big ass dummy load thread!Say hello to my little friend: