Arcam Alpha 1 amplifier (30w) (TIP2955 / TIP3055 output stage)

[ash_dac]

Hi,

Arcam Alpha amp:-

Preamp stage -47K input with 1uf electrolytic input NE5532 / NE5534 with 7815/7915 PSU

Power amp stage:

- BC547 diffrential input, MPSA56 stage
- FST239 / TIP3055
- FST240 / TIP2955

PSU

-1N5403 rectifiers
-torroidal tx (upgraded by me)

Sounds quite nice but seems to take a bit of time (about 40mins) to warm up. Not sure if it's the amp or my cd player !

I don't think there's much I can do to the power section but would it be worthwhile improving the preamp section ?

[hacknet]

regulate the power supply. not many improvments come close to regulating a preamp. i just did it for my nad 302. just wonderful!

[jaycee]

I'd replace the 7815/7915 with LM317/337. Be aware these are not drop-in replacements.

You could try replacing the 1uF electrolytic input caps with some MKS2 polyester parts if they'll fit.

By "warm up", do you mean for it to sound at it's best, or for it to turn on?

[ash_dac]

Originally posted by jaycee
I'd replace the 7815/7915 with LM317/337. Be aware these are not drop-in replacements.

You could try replacing the 1uF electrolytic input caps with some MKS2 polyester parts if they'll fit.

By "warm up", do you mean for it to sound at it's best, or for it to turn on?

To sound it's best it needs some time. It's not that nice to listen to for the first 40mins, and then seems to come alive.

As for the electrolytic caps in the signal path if I go smaller than the schematic I will lose the bass ? (in the schematic 1uf, and 10uf coupling caps are used)

LM317 / LM337 circuit shouldn't be too hard to build on a little board. I probably have enough parts lying around here! +33 /-33V at input of the regulators on the schematic. I have some LT1806, and LT1085 regulators but no negative versions!

Looking at the preamp section there seems to be some kind of FET circuit used after the op amps.

[jaycee]

Yeah, a lower capacitance will raise the lower cutoff frequency.

The issue with taking 40 mins to sound it's best could be related to the bias current. If you have the service manual maybe you should consider re-adjusting this. It might also be a good idea to replace the bias adjust trimmers.

[Johan Potgieter]

I do not know the Arcam Alpha so my remarks must be general.

First, if I understand correctly, then with an input impedance of 47K and a 1 uF capacitor, the -3 dB frequency is only 3.4 Hz! That means that you could lower the value to 220 nF for a 15 Hz low frequency turning point, which places it well within obtainable polyester or polycarbonate capacitors.

But that should not affect your main problem. Here I would agree with Jaycee that it sounds like a bias problem in the sense of warm-up time. You should check there with a multimeter, if you have an idea what it is supposed to be. (You do not mention if this is a problem that developed recently or whether it was always like that.) But it might not be as easily solved as just increasing the bias current. You might also have to check whatever temperature compensating device there is (sensing thermistor/transistor on the heat sink?), and in the end need a circuit diagram with specifications.

As a suggestion, I never switch my own amplifier off. My previous amp remained on for 14 years, barring the odd power interruption (it did not give up after that, it was stolen). The amount of power consumed is negligible and it gives things time to really stabilise.

Of course, if you also suspect your CD player, you should determine whether that is OK.......

Regards.

[pinkmouse]

Ashley, email Arcam for a service manual. They are very helpful, and a full schematic will make modding so much easier.

[ash_dac]

Originally posted by pinkmouse
Ashley, email Arcam for a service manual. They are very helpful, and a full schematic will make modding so much easier.

Arcam have sent me the service manual! Fantastic service

I have the bias figures so I will take a look but I have been working recently on my digital stuff (QED digit dac) . The manual details the bias adjust procedure in full.

I bought it as faulty (dead transformer with paperclip connections ) so it would be a modding project but I've been messing around with digital for ages so haven't looked at the Arcam one bit.

[richie00boy]

I would imagine the pre-amp is for vinyl records only and the line level inputs go straight to the volume control and power amp. So modding the pre-amp will have no influence on the sound. Unless of course the amp has tone controls and you select them to be in circuit.

Check bias setting for your warm up issue.

[ash_dac]

Originally posted by richie00boy
I would imagine the pre-amp is for vinyl records only and the line level inputs go straight to the volume control and power amp. So modding the pre-amp will have no influence on the sound. Unless of course the amp has tone controls and you select them to be in circuit.

Check bias setting for your warm up issue.

Just checked the service manual.

-CD input into single NE5532 amplifier
-this then feeds a NE5534 (one per channel) with tone control section.
-then a balance section
-then a mute Jfet section