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Nelson Pass F5 DIY

Pass Labs F5 Clone

Recently I have been moving away from vacuum tube power amplifiers and more towards solid state amps capable of driving a pair of Electrostatics, namely an older pair of Martin Logan Aerius made around 1995. These speakers have newish panels and rebuilt cross-overs with brand new film caps. However these speakers are a difficult load and quite current hungry. I have also tried these speakers with other power amps, namely a Korsun / Dussun v6i in power amp mode capable of 400W into 4R, an Accuphase P180, a Crown Pro Audio, a few tube amps and finally this F5 which is my main amp at present. This amp was a scratch build using PCB's bought via a group buy in the DiyAudio.com forum. Case is from Modshop in Italy and other parts from RS and CPC.

PCB's and full parts are commonly available on eBay these days if you search world wide. The speaker protection unit is also available from the same site.

 

Build

"The power supply of the tested amplifier is +/-24 volts and should be rated at 6 amps continuous duty, and more than 10 amps peak per channel. Ordinary unregulated supplies will work fine" (Neslon)

I used 2 x 300VA transformers for the PSU and they only just get warm to touch so are not stressed or running near their limit. I know some have built this amp with 500VA per side or even bigger!! Class A amps are very in-efficient, typically around 20% with about 80% of the energy wasted as heat. So as a quick calculation :-

Lets say the transformers are 90% efficient 300VA = 300W @ 90% = 270W Power per tx = 540W
Power Out = 25W at 20% so 80% Loss = 100W Lost as Heat

Total Power Used Per Side is 20/100 x Pt = 25, so Pt = 125W
We need an Heat-sink that can dissipate 100W heat per side.
Total Power Used ~ 125W per side ~ 250W Total

So we are using just under 50% of the Mains Transformer power capacity. Anyhow, that is how I worked out the requirement. Maybe not exact maths or science but close enough for me and the mains transformers run cool. So using a 1KW or 2x 500VA Transformers is a bit overkill IMHO.

 

Speaker Protection

In the images I later removed the protection fuses at the speaker outputs and replaced them with a protection circuit. Fuses create a non linear response and ideally a heavy duty protection relay should be used here. I am not sure how Leak got away with this in their early transistor amps. The protection circuit offers a delay at power-on so the speakers are switched in after 10s. It also disconnects immediately at power off and if it senses DC on the output in a fault condition.

I later removed the protection circuit so there are no contacts in the output audio path.. it sounds better. The circuit has short circuit limiting by design and there is no switch-on-off thumps so the only risk is DC in a fault condition. My risk...

 

Sound

Sound as you would expect from class A is smooth without sounding artificially warm and fat like some tube amps. It has the speed and clean sound of solid state without the grain or haze. Not the biggest and widest sound-stage but ample in a modest room. I have had tube amps with a much wider and bigger sound stage but many tube designs can sound slow, fat and lack control, speed and timing of a good solid state amp on some styles of music. Tubes and Solid State both have their merits.

Considering this amp is only 25W with a current limit at 10A per channel it seems to drive the Martin Logan Aerius ok. It is probably not the perfect power amplifier for these speakers and most people with Logan's seem to use some seriously large and powerful, but very expensive amplifiers. I think in a large room it could be out of its depth on these speakers. I built it because the PCB's became available by chance and it was easy and cost effective to make, esp when you consider that some people throw more cash at a single piece of commercial gear than what my entire Hi-Fi cost which is either 2nd hand equipment or DIY build.

Thanks to Mr Pass for making his design available to the DIY Audio community. Build cost was around £300 in parts including the case.

 

Spec

  • It has very wide bandwidth, DC to > 500 KHz.
  • No capacitors anywhere in the circuit. (except in the power supply)
  • It has a high input impedance – 100 Kohms, and a high damping factor (~60)
  • The distortion is very low, between .001% and .005% at 1 watt.
  • It’s very quiet, about 60 microvolts or so.
  • It will drive a 2 ohm load without burping, and 1 ohm without misbehaving.

    Nelson Pass 5/24/08

 

The top down image with the long green PCB's is the commercial version of the F5 from Nelson Pass for comparison.

 

Upgrades

A year later I made some more upgrades to the amp. I replaced the power supply with better caps and hardwired power supply. See the other article for info.

 

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AttachmentSize
F5-Guide.pdf2.04 MB
F5-Nelson.pdf2.01 MB


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