Maplin Newton pre-amp

[leesonic]

Since I've been looking around this site for information about the Maplin Millenium power amp, I wondered what are peoples views on the Newton pre-amp? I have the original articles for this. Like the Millenium, I like the idea of the PCB mounted components, which to my mind looks tidy compared to the strip wired method.

Anyone running one of these?

Lee.

[Mark Wood]

Well I did build one of these but I've never really used it. I built it without the tone control circuit board and with a change of the final electrolytic coupling capacitors at the final output for polypropylene and a blue Alps potentiometer. There was a warning in the original instructions that on switch on, there's something like 4 V DC at the output and I can vouch for the fact that that's true. Amplify that through a Mission 777 power amplifier as I did and the result was a burned out bass driver in one of my speakers as, sadly, the output fuse didn't blow in time. I contacted Maplin at the time to warn them that this really was a serious issue but the only suggestion was that it should be switched on well in advance of a power amplifier in future, something which I couldn't be guaranteed to always remember! I was also concerned about the loud mechanical hum that came from the mains transformer and after reading Noel Keywood's disappointing verdict on its measured performance, I stuck it in a cupboard where it remains to this day. (I now use a Velleman kit valve preamplifier which I think is excellent value for money and has a relay to switch in the HT after a delay, preventing DC at the output.)
Interestingly, I first used the Newton kit in a rented house where I was very suspicious of the earthing arrangements (or very likely lack thereof) and on trying it recently where I know the supply is properly earthed, the transformer hum appeared to have gone.
I perhaps ought to give it another try. I did wonder if the tone control circuitry was responsible for the poor measured performance (although the RIAA equalisation is pretty crude) that Hi-Fi World found and I'd be interested to hear any opinions on that.
With transformer-coupled valve power amplifiers as I now use, the DC shouldn't really be a problem but I'm very wary of it altogether.

[Mike H]

Yes the tone control is rubbish and should be avoided at all costs.

The phono pre part was OK but desperately needs the cathode follower part to buffer it's output.

The RIAA EQ was originally from Richard Brice's Wireless World design c. 1984, and subsequently judged not accurate enough (though depends how fussy you are). This was revised later. Very important though the 1 Meg grid leak for V2 should be moved to before the network, i.e. after the o/p cap of V1, otherwise the bass is too rolled off as the 1 Meg is directly across the bass boost part which can't then attain its maximum impedance.

Given decent caps and resistors it should be alright. Those in the original were pretty hopeless, e.g. the caps being mains suppressor types etc.

Re the o/p warm-up surge, could do with a transistor clamp or relay with a delay timer but I guess that was making it too complicated

[Mark Wood]

Thanks Mike, that's really useful information. Am I right in assuming you're the Mike Holmes who designed it? If so, apologies for any offence that my comments about the kit have caused(!) as there's no doubt that it was good value for money and did what it was supposed to. At the time, there certainly wasn't anything else to compete with it and at the price, it wouldn't have been reasonable to expect better quality components. (I've always thought the Maplin resistors are OK and a Heathkit AFM-1 tuner I restored using them sounds excellent.)

In fact, Richard Brice put his circuit into one of the articles that he wrote for Hi-Fi World in the early '90s which predate the DIY Supplements and no comment was made then about the RIAA accuracy.

I'm still toying with the idea of turning what I've got into a stand-alone phono stage though. Do you mean that it needs the ECC82 cathode follower circuit on the phono board or an additional one to work properly?

Mind you, I must get on with restoring my Heathkit S-88 amplifier first...

[leesonic]

Mike, do you plan on updating the Newton pre-amp like you did the Millenium power amp? I'm sure there are a ton of people on here who don't know much about valve design, yet are more than capable of putting components together.

Just wish those Maplin PCBs were still available...

[Mike H]

Originally Posted by Mark Wood

Thanks Mike, that's really useful information. Am I right in assuming you're the Mike Holmes who designed it?

Er - yes?

If so, apologies for any offence that my comments about the kit have caused(!) <snip>

No probs, still was all a bit new to us at Maplin back then and also kind of a bit "designed by committee" as well, e.g. not too many valve stages cos of cost, apart from obviously couldn't just copy Brice's design

In fact, Richard Brice put his circuit into one of the articles that he wrote for Hi-Fi World ... predate the DIY Supplements and no comment was made then about the RIAA accuracy.

I've still got a photocopy of a readers letters page showing the revised cct, I think, think it might be from the same HFW, not sure offhand

I'm still toying with the idea of turning what I've got into a stand-alone phono stage though.

That should be alright. As I remember the output level was comparable to my CD player, if using an MM cartridge. Give it some nice carbon resistors like Kiwames or something and proper audio polypropylenes it should be a goer

Do you mean that it needs the ECC82 cathode follower circuit on the phono board or an additional one to work properly?

Yes the same one, that's what it was for mostly. Just had a volume pot in front of it so can also be used like a line buffer with inputs selector switch

Originally Posted by leesonic

Mike, do you plan on updating the Newton pre-amp like you did the Millenium power amp? I'm sure there are a ton of people on here who don't know much about valve design, yet are more than capable of putting components together.

Ah well it would be quite different. Have tried various tone control topologies (topologies?) since, to greater or lesser degrees of success, it's surprisingly not as straightforward as might be supposed! I am currently looking at a very old Wireless World design which looks very promising in principle. The original article describes a "conversion" to add bass and treble controls, but without adding more valve holders, to an otherwise completely populated chassis (i.e. no space left or too complicated making new holes). The idea is that some single valve position is replaced with a double-triode (still only one valve holder) having the tone controls in between. Quite ingenious really.

Without giving too much away my current idea therefore is a SRPP input stage to drive the network, which is then followed by a transformer based output stage which will also be able to run headphones directly.