Amplifier listening tests and comparisons: Find - or design - a sophisticated RCA/XLR amplifier switch box?

[Xulonn]

I've been able to find input selectors with attenuators (passive preamps?), and speaker selectors, but not an amplifier selector with the features needed to facilitate level-matched amplifier comparisons. I have not found a commercial product for sale that has the features I want, I cannot think of any other way to compare amplifiers properly via instantaneous, level matched comparisons - preferably double blind.

Basically, the concept is to switch either XLR or RCA between amplifiers, and simultaneously switch the selected amplifier to a pair of speakers, with the circuit designed to protect all components from shorts - and tube amps from open, no-load speaker connections. Also, some amplifiers - like my old Classé have internal switching to select the XLR vs RCA inputs, so I decided that it would be a more versatile design to do RCA vs XLR testing via the switchbox.

For me, the ideal switch-box would accept a single-source (a DAC in my case) signal via either RCA or XLR, and route it to one of two amplifiers via either XLR to the box, then XLR to an amplifier, or RCA to the box and then RCA to the amplifier. Or one could compare an RCA-connected amp with an XLR-connected amp.

Indeed, in my case, I could use my Topping DX7s to feed both the XLR and RCA inputs of the switch box simultaneously, and I could even use the box to compare an XLR input only amp with an RCA input only amp.

For level-matching, there would need to be a pair of high-quality stereo attenuators - one on each pair of inputs - and perhaps even a level-matching meter on the front panel. Assuming that testing would be at medium to loud levels, the common problem of low-level channel imbalance with attenuators using continuously variable wipers - rather then discrete resistor elements - should not be a problem.

In the same box, and switched at the same time, would be the amplifier to loudspeaker connections. It would be best to switch the two amps between the loudspeaker and a dummy load, because no load and high gain can apparently damage tube amps due to shorts occurring in the output transformers.

I haven't drawn a schematic in years, and never was an expert, but this might be fun to try. I did an internet search, and was blown away by what I quickly found at CircuitLab.com - a free electronic schematic circuit builder that is extraordinarily simple and intuitive. Looks like more fun coming up for this old audio and electronics hobbyist. The principle is simple as far as electronics go - a box/case, some RCA and XLR jacks, good switches and attenuators, and speaker binding posts.

First questions:

What would be a good resistive/inductive dummy speaker load (type and ohms) that would protect amplifiers without getting too hot?

What are the parameters for input attenuators for such a device?

 

[Xulonn]

I just sent a request for quotation to Paul Baldwin of ReelAudioCo in the UK for a variant of the custom switch box in the preceding link and pictured below:

Here is some of the content of that email:

Recently, I purchased a Musical Paradise single-ended KT77/EL34 amplifier, and would like to occasionally listen to it instead of my vintage Classé 70wpc solid-state class AB amplifier. Switching amps manually is a pain in the arse, so I went looking on the internet for an amplifier comparator with the features I wanted, and although one of Frank Beresford's units came close, it wasn't quite what I wanted - and he wasn't interested in a new design or a custom build.

Then I found a post by a gentleman named "Edward" at TheArtOfSound.net about a custom switch [above photo] made by you for 2 amps and 2 speakers. It appears to be properly designed and built, so I would like to get a quote on a variation of that design. I used your "wish list" for Edward's unit as a template for my own wish list, which is listed below.

I asked Paul for a quotation for 1, 2, 5, and 10 units, and suggested that I would promote the comparator here, and if we can get enough orders for the discount levels, we could receive a discounted price.

Amplifier Comparator Wish List:

- A box to level match and switch between a pair of stereo power amplifiers - 1 valve and 1 solid state

- Design for one pair of speakers, no speaker switching necessary, but dummy loads are needed to protect the tube amp.

- The box would contain 2 separate sections that switch simultaneously:
1. Line input from DAC to amplifiers with optional high-quality potentiometers for level matching
2. Switch between dummy load and speakers to send active input to speakers and no signal to dummy load.

- There must be no audible switching noises or distortion

- The Switch Box should be capable of switching up to 200w per channel.

Requirements - Line Level Input:
- 1 Pair line level XLR input from DAC

Switch between:
- 1 Pair line level XLR output to SS amplifier
(Many good amplifiers have XLR connections - and XLR>RCA adapter cables are available)
- and -
- 1 Pair line level RCA output to valve amplifier
(Very few vacuum tube amplifiers have XLR input connections)

Connections - Speaker Output:
- 2 pairs of 5-way red (+) and black (-) speaker connections.

Question: Common Earth for all negative amplifier and loudspeaker connections? Is this correct?

 

[somebodyelse]

If it can have a remote control input of some sort the random element of the ABX,and the scoring, can be automated so you don't need an assistant. The presence of large relays reminds me of a problem described in one of the papers on blind testing - some of the participants reported being able to tell which way the system was switched because of slight differences in the sound made during switching.

 

[Ilkless]

https://avahifi.com/products/abx-switch-comparator

Have you looked at the Van Alstine ABX comparator?

 

[somebodyelse]

It doesn't look like the Van Alstine would do the dummy load part of the requirements, although it's not entirely clear.

 

[Speedskater]

How does a dummy load enter into a listening test?
(tube amps excepted)

 

[somebodyelse]

How does a dummy load enter into a listening test?
(tube amps excepted)

It doesn't, tube amps excepted.

 

[Xulonn]

https://avahifi.com/products/abx-switch-comparator

Have you looked at the Van Alstine ABX comparator?

Out of my price range, and it has more features than I want, with no mention of dummy loads for tube amp protection, as others have mentioned.

I've gone back and forth with Paul, and decided to go RCA single-ended only. When he finalizes the circuit and quotes prices, I will post again.

Since there is a 1/2 second delay in the relays for an extra margin of safety, it may not be ideal for ABx testing.

 

[robjamell]

I have developed a prototype wireless remote control A/B speaker switcher. It has a wireless remote with one button. You just push the button and it switches from A to B, next push back A, etc.. There is a corresponding LED on the box for A or B. I close my eyes and after I go back and forth, I open them to see which I prefer. I also use it in reverse to compare amplifiers. There is no delay when switching.

I have seen the need for it to be affordable.

I am trying to determine how much these could be sold for if I was to produce some basic hand-made pieces. Just as you guys said, the Van Alstine is nice, but probably not affordable. In turn, mine is way more down and dirty, but takes quite a bit of assembly time.

At what price point would you guys be interested?

Thanks

 

[Blumlein 88]

You really don't need anything special for the tube amps though it would be convenient. Sticking a 100-150 ohm 10 watt resistor across the output posts will take care of them just fine. So if you did this the switcher won't care, and the amp will be happy enough when the full low resistive load disconnects. With the normal speaker connected the 100 ohm resistor won't effect anything enough to matter.