Jimbos Audio ramblings

Sometime back I was looking at replacing the Croft 7 power amplifier with a few alternatives in order to gain some more power and indeed I tried using Croft Series 7R monos but they didn't do the trick. I also tried a Neurochrome 686 but preferred my Croft 7. The Neurochrome was quoted with way more power but certainly didn't sound it in practice? I did borrow an old SS amp off a friend but that was a bit slow and veiled so I had thought I may try a Class D and the only amp I really thought might sound great and not just give me lots of power was the Purifi Eigentakt Power amplifier. I have been interested in this amp for a long time as indeed it does seem to have had a great reputation from its inception.

There are a number of different manufacturers who build the Eigentakt power modules into an amplifier and to be fair it hard to do it cheaper yourself!

But for anyone interested in building their own Purifi amplifier here is an excellent video tutorial. No soldering needed so very straight forward build

https://www.youtube.com/watch?v=mMmkK8jisDg

These chaps also do some great videos comparing class D amps including the purifi which are quite interesting.

The 686 power will depend on the build, and what the input voltage is.
If it is a “safe and sane” build, you are looking at about 120w into 8ohms. You need to up the input voltage to get the max power output. I wanted mine to run relatively cool, so went for a reasonably “safe and sane” build.

the Krell amp I use lights up an LED if it outputs more than 0.75 of a watt for more than two seconds

You've really got to cane it to get it to light up (with large 90dB speakers in a large-ish room).

Voltage regulation matters more - if the speaker load is demanding - unless using insensitive speakers.

Never hurts to have more watts but usually not necessary. Plus double the watts power is only 3dB 'louder' which is noticeable but not a lot.

There's watts and there's watts. In my experience the quoted output for an amplifier does not always translate into what you hear and how it drives your speakers. I have heard some low powered valve amps that sounded way more powerful than they were and I have heard SS amps that had a huge quoted power output but you would not have guessed when you listened to them.

I have heard some Monster power amps - 750W monos and they certainly gripped the drive units and thumped your chest. Bizarrely I heard some Croft low powered OTL mono power amps and they gripped and controlled the speakers unbelievably and they were only 10W each!


Martin my experience of Krell amps is they do what they say on the tin!:)

well it's all about the combination of speaker impedance curve/phase angle and sensitivity and efficiency of the loudspeaker - it's a complex relationship - in the real world you have to just try it and see (or hear) how good it works, paper specs only a guideline.

with a valve amp using output transformers it's even more of a lottery since you can get frequency response variations as it interacts with speaker impedance. This could make a valve amp seem more powerful depending on that interaction.

I'm personally not much for valve power amps (unless OTL) since you really do need to choose the speaker very carefully to get an accurate response and for me that's the tail wagging the dog. I rather choose speaker then get an amp to drive them.

Heard too many systems with valve power amps and you get that bloat or even boom in the bass which ruins the whole sound - although some seem to like 1950s radiogram bass.

Most speakers have a typical efficiency of about 5%. Thus if you want to achieve an SPL of 99dBm at 1m distance from the speakers, then you need about 1W of power.

Horn loaded speakers are much more efficient, which is why they can be used with amps offering single figure output power.

Whereas electrostatic speakers are much less efficient and need correspondingly greater amplifier power to achieve the same SPL.

I tend to agree with you there regarding power amps. In general I would say that most of the all valve power amps I have heard are a bit "uncontrolled" in the bass. My power amp is not really a valve power amp. It is an OTL hybrid with MOSFETS doing the work of the output valves so it behaves a little more like a SS with only one valve on the input stage.


I have heard so many SS power amps over the years and so I have a good handle on what they can and cannot do. Everything from Ultra linear, total control, neutral sounding to Warm, fat almost tube like sound. There are many chocolates in the box to pick from depending on what you want to achieve but for me it has to be foremost an engaging sound that is not fatiguing and provides plenty of detail and warmth. I don't like neutral, flat sounding amps that are supposedly truthful. I put the Krell amps in the warmer end of my category and they are certainly engaging and have plenty of drive.

I initially dismissed Class D amps as a possible replacement for the Croft as they had so many aspects about them I would instantly dislike but I heard a conversation about the Purifi amps that suggested they had some aspects about them that were similar to valve amps but without the valve amp negatives and indeed they have the ability to control and drive larger speakers with great control especially the bass which is why they have been used for many years in active speakers plus their size and limited heat output. The only question would be would a Purifi sound as live and engaging and dare I say as musical as my Croft?

The purifi amplifier can be had for very little money and this would where I would start. Try and build that yourself for that price?

https://nordacoustics.co.uk/product-...urifi-1et400a/ I am looking at the Nord 3 stereo power amp at £1099!

IIRC Gordon Halftree had some Nord multi-channel amplifier powering his Linkwitz speakers at NEBO a couple of years ago, Sounded fantastic, which may have been down to the speakers, but obviously the amp wasn't letting the side down. And Gordon knows his stuff...

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Originally Posted by Pigmy Pony

IIRC Gordon Halftree had some Nord multi-channel amplifier powering his Linkwitz speakers at NEBO a couple of years ago, Sounded fantastic, which may have been down to the speakers, but obviously the amp wasn't letting the side down. And Gordon knows his stuff...

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I have heard Gordons system Steve and it is stunning. His multi amp approach and system set up is something else as is the sound.

Quote:

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Originally Posted by Macca

well it's all about the combination of speaker impedance curve/phase angle and sensitivity and efficiency of the loudspeaker - it's a complex relationship - in the real world you have to just try it and see (or hear) how good it works, paper specs only a guideline.

with a valve amp using output transformers it's even more of a lottery since you can get frequency response variations as it interacts with speaker impedance. This could make a valve amp seem more powerful depending on that interaction.

I'm personally not much for valve power amps (unless OTL) since you really do need to choose the speaker very carefully to get an accurate response and for me that's the tail wagging the dog. I rather choose speaker then get an amp to drive them.

Heard too many systems with valve power amps and you get that bloat or even boom in the bass which ruins the whole sound - although some seem to like 1950s radiogram bass.

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Yeah the technical aspects of hooking up amps to speakers regarding impedance curve/phase angle, sensitivity/efficiency etc is very complex when trying to find the perfect amp combination and relying on technical measurements alone simply does not work. Suck it and see is really the only way you are going to know.

Throw in room interaction and your into another whole level of pain.