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View Full Version : FS: Radford ZD100 power amplifier, ‘Zero Distortion’ range, quite rare


montesquieu
04-03-2017, 19:27
Been having quite a bit of fun with this, indeed it’s probably the best solid state amp I’ve ever heard in my system (and that includes a Krell KSA50). But now it's time to pass it on.

Radford ZD100 power amp from the mid-70s, four mosfets per channel. The specs claim a truly astonishing 0.004% maximum total harmonic distortion at both 1khz and 10khz, qualified as ‘typically' 0.0015% which I suspect is barely within the limits of measurement. Intermodulation distortion less than 0.01%. The ‘zero distortion' tag comes from the fact that the entire range, power, pre and integrated, shows measured distortion below circuit noise level.

Power is 90w into 8 ohms, 150w into 4 ohms. Input sensitivity is 1V so unless your sources are fairly high gain, it probably needs a preamp with some gain in it rather than a passive (it sounds very good indeed with the EAR 912).

How does it sound? Well actually, quite a lot like a Radford valve amp. Natural rather than exaggerated or artificial highs, detailed but also punchy midrange, the biggest difference is in the bass quality which is stunning, in terms of both grip and depth.

I really ought to keep it, but I have three power amps at the moment (Radford STA100, EAR 861 and this one) and I don’t really need three.

Comes (amazingly) in its original 70s box (I have an outer box sorted for it to protect this)with both a printed multi-page data sheet and the technical instructions/installation and advanced maintenance booklet. Time-warp condition with barely a mark on it, and freshly serviced in February (including some parts replaced and rebiasing). There probably isn’t a better one out there.

Probably suit a valve lover who needs more welly than valves can generally provide without getting all bulky and hot.

I’m looking for £450 including courier for this, £425 if you can collect from Bracknell area. It's a crazy sonic bargain at that.

http://i1181.photobucket.com/albums/x440/montesquieu61/image6_zpskxkwembb.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image1_zpss4jxwusv.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image2_zpssc7iyvcr.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image5_zps274oqb4x.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image4_zpsdnn3s2hk.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image3_zpsaje46dgl.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image7_zps3gkpston.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image8_zpsniwqvmgt.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/image9_zpsjpegdtas.jpg
struth
04-03-2017, 19:38
Lovely. I dont use a power and i still want it.... this shouldnt take long to walk.
nthall
04-03-2017, 19:46
I want it, but I'm on the wrong side of the Atlantic.

Sent from my Z981 using Tapatalk
mikeyb
05-03-2017, 08:00
Any thoughts on how this would sound with a Croft Epoch?

Visually it would match nicely but never having heard one I wouldn't have a clue if it's a good match sonically. My PMC 21's like a bit of power too so not sure if the 90w here will be more beneficial than the 72w from my TQ Claymore.
montesquieu
05-03-2017, 09:42
Should be a good match with the Croft.

Haven't heard the Claymore so can't really comment on that. I suspect the design approaches are very different so you should hear a difference anyway!
vinyl
05-03-2017, 11:33
Have you mentioned this to Will at Radford Revival? I'm sure he'd be interested in this rarity.
cuddles
05-03-2017, 23:14
Ooh...I'm tempted.
Audio Al
06-03-2017, 03:42
Tom , any inside gubbins photos :eyebrows:
montesquieu
06-03-2017, 11:43
Will see what I can do ... I generally leave the lids on because 1) all the fairy dust that makes them work might leak out, and 2) I'm a total klutz and might electrocute myself.
Audio Al
06-03-2017, 12:03
Will see what I can do ... I generally leave the lids on because 1) all the fairy dust that makes them work might leak out, and 2) I'm a total klutz and might electrocute myself.

:scratch: unplug it and keep body parts outside :D
spendorman
06-03-2017, 14:17
Probably looks a bit like this, my ZD50. Apologies, pics taken about 12 years ago and not good.

https://c1.staticflickr.com/4/3684/32902587000_888a93eeb6_z.jpg (https://flic.kr/p/S8umNL)PICT0011 (https://flic.kr/p/S8umNL) by A60man (https://www.flickr.com/photos/41679262@N02/), on Flickr

https://c1.staticflickr.com/4/3696/33285707645_3e10c19a99_z.jpg (https://flic.kr/p/SHkXe8)PICT0009 (https://flic.kr/p/SHkXe8) by A60man (https://www.flickr.com/photos/41679262@N02/), on Flickr

https://c1.staticflickr.com/3/2816/32902588360_ff50051cf6_z.jpg (https://flic.kr/p/S8undd)PICT0010 (https://flic.kr/p/S8undd) by A60man (https://www.flickr.com/photos/41679262@N02/), on Flickr
montesquieu
06-03-2017, 14:33
Actually there's quite a bit more 'gubbins' it has double the number of output devices on the back (not including another two each side, input devices I suspect) and a rather fuller case.

I note that the mains transformer appears to be configurable for a range of voltages from 100v-120 and and 200-240.

It's bloody heavy for a solid state amp as well (weight made up of components rather than a fancy faceplate too!)

http://i1181.photobucket.com/albums/x440/montesquieu61/zd-inside2_zps24yrledu.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/zd-inside1_zps3xlwgojh.jpg
spendorman
06-03-2017, 14:38
I think that the circuit is basically similar, apart from the extra power transistors, higher supply rails etc.
Arkless Electronics
06-03-2017, 15:27
Actually there's quite a bit more 'gubbins' it has double the number of output devices on the back (not including another two each side, input devices I suspect) and a rather fuller case.

I note that the mains transformer appears to be configurable for a range of voltages from 100v-120 and and 200-240.

It's bloody heavy for a solid state amp as well (weight made up of components rather than a fancy faceplate too!)

http://i1181.photobucket.com/albums/x440/montesquieu61/zd-inside2_zps24yrledu.jpg

http://i1181.photobucket.com/albums/x440/montesquieu61/zd-inside1_zps3xlwgojh.jpg

Voltage regulator series pass devices. It and the 50 have fully regulated supplies a la Naim NAP250.
montesquieu
06-03-2017, 18:27
Have you mentioned this to Will at Radford Revival? I'm sure he'd be interested in this rarity.

Will tells me he has two already or would definitely be interested :)



Voltage regulator series pass devices. It and the 50 have fully regulated supplies a la Naim NAP250.

Thanks Jez - assume that's a good thing then! (I'm not really familiar with Naim).
Radford Revival
06-03-2017, 18:45
Will tells me he has two already or would definitely be interested :)

I once used one to drive a pair of 25Hz TL subwoofers and the other to drive a pair of two-ways, actively crossed over at 100Hz or so. That was a loud system!
montesquieu
07-03-2017, 09:59
Sold and will be powering 15in HPDs so just about perfect I would think!
Audio Al
07-03-2017, 11:05
Sold and will be powering 15in HPDs so just about perfect I would think!

See what happens Tom when you take your clothes off :eyebrows: