Arcam Alpha 9 integrated amplifier

[audiocrow]

I'm wondering to "tweak" my Arcam Alpha 9 amplifier. Does anyone already made it? Have ideas??

I already made:

-replace the tl072 (pre)+ ne5534(dc servo)...I think that's their functions... with opa604 series.

- add 100nF Philips bypass capacitors in Mains Capacitors, diode, opamp supply pins

- replaced original 100mF with 1000mF capacitor used in opamp regulators

- ferrite in toroidal cables

- some chassis damping

[JonHarrison]

audiocrow,

I have a late model Delta290 which appears very similar to the Alpha9 (if not identical).

I'm going to try the following ...

PSU
1. Put transient suppressor (and 1nF to gnd) before transformer.
2. put 8R/17W + 220nF in series before transformer.
3. put 2 x 100nF polyproplene 100V+ before bridge.
4. Replace the main caps with 4x10000 or 2x22000 63V (longer life than the 50V fitted).
5. Put 1ohm/17W in series with 680nF at the end of this lot.
6. Try a second bridge, so one for each supply rail.
7. Provide seperate trafo for the +/-15V supply
8. Replace the 317/337 with lower noise regulators.

PRE-AMP
9. Replace the tone-control NE5532 with OPA2134.
10. Replace TL072 with OPA2134.

POWER-AMP
11. Add 0.22uF across the zeners to shunt out noise
12. Think about removing the DC servo and replacing with a good 2.2uF cap or replace the opamp with a AD8620, decoupled with 100uF + 100nF X7R on each supply pin (or OPA627).

Could also replace NE5532 with class-A biased OPA2604, replace DCservo NE5534 with OPA134.

Have you tried anything else ?

Cheers,

Jon

[audiocrow]

Thank you!


some questions about your tips:

PSU
8- wich regulators?

PRE-AMP
9- i've replaced tl072 with opa2604 Class A biased...does opa2134 will improve my current performance?

Power-AMP
-dc servo now using opa134...cristal clear does AD8620 will improve?

[JonHarrison]

João,

>Which regulators ?

Regulators, I hadn't really given much thought to. I'd probably go for a low noise Linear Technologies adjustable regulator and then reuse the 317/337 as a pre-regulator. Also, I'd probably try and seperate the supplies to the pre-amp and gain stage. From what I remember the uP control has a discrete regulator. Perhaps I'd do some work to ensure that any HF noise present doesn't come back and polute the other supplies. I'd start with a scope and spectrum analyser and actually look at the supply rails first though. Can you switch the display and remote circuitry off on the Alpha 9 ?

>I've replaced tl072 with opa2604 Class A biased...does opa2134 will improve my current performance?

I haven't actually listened to the OPA2604 or the OPA2134 yet so I am not really in a position to comment. However, on paper, the OPA2134 is superior, and people who have used it in similar applications seem to be pleased with the results so why not give it a try and see what you think.

>dc servo now using opa134...cristal clear does AD8620 will improve

If you're happy with the OPA2134 than stick with it. The AD8620 will inject less noise back into the signal path. It also has a higher slew rate and faster settling time so it should be better in that respect as well. It's not cheap though !

Regards,

Jon

[richie00boy]

A warning on modding the PSU with twin bridges. Unless the transformer has twin secondaries then you are wasting your time. If it's just a centre-tapped type then half of each will be shorted out if you put two in.

Otherwise, mods sound good. I wonder how you have arrived at the component value for your 8R/220nF and 1R/680nF snubbers? Also, unless you use non-inductive (i.e. not wirewound) resistors you will simply be adding more unwanted inductance. I'm not sure you need such high wattage either.

[JonHarrison]

Hi richie00boy,

Thanks for the advice on the bridges.

I've just taken the lid off and it doesn't have split secondaries, so as you say a twin bridge here would be a bad idea.

The snubber values were just taken from the TNT Solid State Power Amplifier Supply ( http://www.tnt-audio.com/clinica/ssps2_e.html ). The values probably aren't optimised for this supply but ok as a starting point and better than nothing (I hope). When I get around to actually tinkering with the amp I'll measure the leakage inductance of the secondary and inter-winding capacitance and have a go at calculating some better values from that.

Cheers,

Jon

[Evaas]

I've never seen an arcam circuit, but I'm wondering if they have anything unique in there? Arcam always sounds a little different to me (colored), and I tend to like the sound. I'm curious how thye achieve it.

[SimontY]

Originally posted by Evaas
I've never seen an arcam circuit, but I'm wondering if they have anything unique in there? Arcam always sounds a little different to me (colored), and I tend to like the sound. I'm curious how thye achieve it.

I heard the 8 and 9 once. To me, it was like a duvet covering the speakers. Not surprised they moved to the DIVA range.

With this in mind, it might not be a good idea to damp the chassis! And by the same token, think carefully before increasing psu capacitance. Why not tune the sound with cones or soft feet? What's your preference?

Best of luck whatever you do, I love to see people upgrading ordinary commercial kit

[jaycee]

Arcam seem happy to give out the service manuals.

The Alpha range was N-Channel MOSFET output based, pretty conventional asymmetric design with current mirroring. The outputs are IRF540's and the rest of the transistors are BC547/557 and BC546/556. The preamp uses 4560's capacitively coupled.

The DiVA range uses an opamp frontend which drives a pair of Sanken SAP15 darlingtons using a Wilson current mirror that picks up the changes in the supply rails of the opamp, which drives a constant resistive load. The opamp is only a TL072 so maybe replacing this would bring a great improvement to this design (the preamps are OPA2134's though). I think the biggest flaw in this series is the tone control section, which uses Dallas digital potentiometers and limits the signal to 2.5v swing.

[Evaas]

Originally posted by SimontY

I heard the 8 and 9 once. To me, it was like a duvet covering the speakers. Not surprised they moved to the DIVA range.

hmm - that's how I feel about classe, but not arcam. I wouldn't say they're the best I've heard, but they have a great euphoric sound. the pass aleph is the only amp I've heard that can drive my speakers, but the arcam was the closest runner up.

Originally posted by jaycee
Arcam seem happy to give out the service manuals.

The Alpha range was N-Channel MOSFET output based, pretty conventional asymmetric design with current mirroring. The outputs are IRF540's and the rest of the transistors are BC547/557 and BC546/556. The preamp uses 4560's capacitively coupled.

The DiVA range uses an opamp frontend which drives a pair of Sanken SAP15 darlingtons using a Wilson current mirror that picks up the changes in the supply rails of the opamp, which drives a constant resistive load. The opamp is only a TL072 so maybe replacing this would bring a great improvement to this design (the preamps are OPA2134's though). I think the biggest flaw in this series is the tone control section, which uses Dallas digital potentiometers and limits the signal to 2.5v swing.

interesting. maybe it's the mosfet sound that I like. I haven't heard any of the DiVA's but they seem a step down to me. Maybe if the opamp is good and layout is great then it will sound good

Evan