Amplification Best Approach for Efficient Active Speakers

vinylnvalves · Post by **vinylnvalves** » Fri Sep 05, 2014 12:43 pm

My statement regarding measuring transformers was based on the following ..

Discussing it with a "friend" with electronics testing equipment has confused me further as he disagrees with the statement of not using a digital meter, but using a analogue meter. He says that digital meters have 10 M Ohms on the probes or more, so should be less risk than an old AVO.
Is this a real concern to worry about? Who is right - has some knacked a transformer measuring it?

Alex Kitic · Post by **Alex Kitic** » Fri Sep 05, 2014 8:59 pm

I don't believe you can destroy a transformer by measuring it with a multimeter!

That would be a very fragile unit... what can you destroy, the winding?

As for the amps, it all depends on the power needed. There is no particular need to apply DHTs (or DHPs) unless for the sake of doing it (l'art pour l'art). Keep it simple, indirectly heated tubes have less requirements and sound just fine, depending on the tube and circuit. At least it does not matter whether you'll use DC, AC, or HF AC (although the latest I've heard on a forum is that HF AC is particularly troublesome with indirectly heated tubes due to the "diode effect" ... no comment).

On the other hand I believe that matching the amps in a multi-amplified system is extremely important, thus you need predictable results and the same topology throughout. Accidentally, the RH series might prove useful, but of course you could pursue a different course with some exotic amps using interstage transformers and DHT tubes, possibly very rare NOS, and unobtainium socket type (just kidding)

Alex Kitic · Post by **Alex Kitic** » Fri Sep 05, 2014 9:12 pm

You should have told us the info is from the JAC site?!
That changes the story from "potentially alarming" to "a joke".

While it is a fact that people can find ways to destroy anything, this is about magnetizing the core and thus making the transformer sound bad! Really, if you magnetize the core, how long is it going to stay magnetized? Forever?!

This guy is clearly exploiting the "audiophilia nervosa" syndrome.

brig001 · Post by **brig001** » Sat Sep 06, 2014 9:04 am

Why not test them using AC? You can't check for shorted turns with a multimeter anyway.

Connect the primary to a headphone out on something (PC or MP3 player) and connect the secondary to the input to an amplifier. Keep the levels low and the gain low to start with and see how they sound. If you have test tones in the hundreds of Hz, and your meter will measure AC mV you can measure primary and secondary voltages to check the turns ratio is about right. If you want extra protection for everything, put a pot before and after the transformer and turn them both to zero before you start.

HTH,
Brian

rowuk · Post by **rowuk** » Sat Sep 06, 2014 10:14 am

Well we are on page three and are we closer to ideal amplification? I think not. We are already onto hardware with no parameters about what is really needed.

When we attack a problem randomly, the results match that. If we carefully plan the use case, we can get much closer the first time around.

How big is the room where the system will be set up?
How loud do you want to listen
What do you want to listen to
Are you pretty firm with the drivers that you chose?

Then we have at least the power requirements. It makes a big difference if we are planning SETs or megawatt SS.

After that then we can talk about "sound" quality.

As far as magnetizing the OPT cores, we have hundreds of direct coupled DC volts later in the amplifier - not always with music playing - what can a multimeter do to transformers designed for that?

Sounds like a great scam to sell bogus consulting services. Surely a company to avoid doing business with.

Alex Kitic · Post by **Alex Kitic** » Sat Sep 06, 2014 3:41 pm

I have understood so far that these amplifiers don't need to be particularly powerful. Maybe I got it wrong?

On the other hand, I don't expect this thread, like most other threads, will result in some amps being built... I consider it a "what if" discussion.

vinylnvalves · Post by **vinylnvalves** » Sun Sep 21, 2014 9:04 pm

Now that i have a bit more time - i am starting to move forward. Amps will be built - i have just been distracted getting my analogue sorted.
Have had some other inputs into the mix which i have been considering. To one of the questions i can play the JBL's 2482 very loud off a Ipod, so only need milliwatts there - but must be no hum. The second amp for the beyma TPL's is the first one i will build first, as i have most components except for the OPT's. I have been recommended to look at the Sowter 9950's as they are designed for HF. Anyone any experience of these or something similar?

vinylnvalves · Post by **vinylnvalves** » Sun Nov 23, 2014 10:36 am

Just measured the smoothing chokes i am intending to use, with a borrowed LCR meter and they only measure 3.8 H. I was convinced that they were 10H ones due the size (on a 105 stack). There is obviously some effects on the frequency at which you measure at. I have in the back of mind that a smoothing choke will measure low, when using an LCR meter thas working at say 1200Hz.

An i kidding myself that they are 10H or is there something in the measuring of smoothing chokes?

pre65 · Post by **pre65** » Sun Nov 23, 2014 10:50 am

As I understood things it depends on the current.

For a more technical explanation see this.

http://ieeexplore.ieee.org/xpl/login.js ... %3D1669991

IslandPink · Post by **IslandPink** » Sun Nov 23, 2014 11:47 am

From memory when I've been quoted specs on output trannies by Bud Purvine, the inductance at 20Hz can be higher than eg. 150Hz , but the change is small, maybe 10 or 15% at most . I guess ( as Phil linked ) you are OK regarding DC on testing as it's just the meter ?
Looks like they are most likely less than 5H . Perhaps they're gapped for more DC than you thought ?

vinylnvalves · Post by **vinylnvalves** » Thu Dec 04, 2014 7:16 pm

I need to source a mains transformer for the 4P1L amp. I am needing around (25+36+36) 100ma per channel @ 280V, so a minimum of a 100VA transformer i think. I think i maybe could get away with a 240V isolation transformer, but am thinking a may like a few volts in hand in case i wanted to add voltage stabilisers to the output stage in the future, and its always easy to convert a few volts to heat!

I have seem this on ebay http://www.ebay.co.uk/itm/161361232848? ... EBIDX%3AIT Using the 200V input tapping i reckon i could get out 260V (AC) out if i combined the outputs. There is something nagging me in back of my mind that i cannot combine the outputs. Can someone tell me whether i am being stupid.

vinylnvalves · Post by **vinylnvalves** » Fri Jan 16, 2015 8:58 pm

After considering what i am going to be driving, i have decided to go for a SE instead of a PSE output stage. I have attached one of Andy designs that fits the requirements. As i have most of the bits now to make this excepting the regulator tubes, i throught i would build a "plywood" amp to start with. The only worry i have is noise, i have been warned that i could not use this amp to drive my JBL 2482's, and it may be marginal with my 98 db efficient Beyma TPL's. ( Andy use 89 db Alpairs -so no issue)

The advice i have had for improving the noise floor is as follows...
I would suggest using the Coleman regulators and losing the filament bias , which is extremely inefficient and also places the filament supply in series with the grid . You can either use grid bias or cathode bias instead . With 4P1L , it's best to back off the DC to approx. 3.8V in my experience . This is a way of improving microphony at the small expense of a little higher Ra

I have also had contridactory less reliable advice, so i am wanting to know has anyone else used this tube in a similar application, and if so did you have noise/microphony problems.

simon · Post by **simon** » Sat Jan 17, 2015 12:16 am

This is on my (long!) list of amps to build. There's a great deal about this valve on DIY Audio (as I'm sure you know) and whilst I can't comment as I haven't built it Andy is passionate about using filament bias as he finds the cathode resistor bypass caps has a significant deleterious affect on sound. Coupling caps too, hence the IT.

Ale on DIYA has reported good success with reducing microphonics by current starving the filaments too, easy enough with Rod's modules I guess. Not sure why the focus on the noise floor - is it because of the DH driver or is there something else? Lack of cathode bypass cap? I have a 3 stage DHT amp (with Rod's modules) and they're fine with my 97dB speakers, but the cathode resistors are bypassed.

pre65 · Post by **pre65** » Sat Jan 17, 2015 11:09 am

simon wrote:This is on my (long!) list of amps to build.

Me too.

I'd let them go if anyone is interested.(set of 4).

vinylnvalves · Post by **vinylnvalves** » Sat Jan 17, 2015 3:20 pm

i think it was more microphony than noise, especially with my compression drivers @ 114 db eff. I believe these tubes are particularly bad for microphony - mechanical isolation i can do by mounting the sockets on iso mounts, may try and get some PTFE loctals as they are better than ceramics.

As i am only using the amp above ~1000Hz being feed by DSP whether cathode bias would be "good enough" as the coupling capacitor could be quire small.

audio-talk

Amplification Best Approach for Efficient Active Speakers

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