ImPasse Preamplifier

[KatieandDad]

Also, as I'm no expert on valves, I've ordered a Russian NOS 6SN7 (£12 for two) and Brimar ECC88 (£12 for two).

I notice that some 6SN7's are about £50 each !!!

Yes. I know I only need one of each, but spares are always useful in the valve world.

 

[SY]

Why are R3 and R10, the grid stoppers, specified as Carbon Composition?

To quote Tevye, "Tradition!"

I have some exotic 6SN7 (5692, CV1988), but the EH measured just as well.

 

[willamp]

To quote Tevye, "Tradition!"

Thanks for the confirmation. That's the only reason for the carbon grid stoppers I've been able to come up with.

 

[KatieandDad]

I was always lead to believe that carbon were noisy and metal film were better ?

 

[SY]

There's no current running through them and they are low resistance value, so the noise contribution is negligible. Remember, that stage is fed through a potentiometer, which will have an order of magnitude more resistance.

 

[willamp]

There's no current running through them and they are low resistance value, so the noise contribution is negligible. Remember, that stage is fed through a potentiometer, which will have an order of magnitude more resistance.

I agree, also their exact value isn't important so the fact that they change value with temperature and time doesn't matter. I don't have any on hand so for me it's extra trouble to seek them out just for grid stoppers. Because of this I don't bother. Easier to just buy metal film like everything else. I always thought another reason to use them for grid stoppers was just to get rid of the stock on hand.

 

[KatieandDad]

OUCH - The postage for the Mains Transformer is $249. The transformer was only $29. Still, better to get the right component, the only UK spec component I could find was for a power amp with a King sized price tag.

 

[SY]

That's a ridiculous price, it's a cheap transformer. Source something more local or from someone who knows how to get product into Europe at a reasonable cost (Antek, perhaps?).

 

[KatieandDad]

I'm looking at how to match R11 and R12 to +/- 0.1%.

With a standard DMM the DMMs resolution isn't fine enough.

15K +/- 0.01% gives us 14.985 - 15.015K

On the DMM I could measure 14.98 to 15.01K

 

[KatieandDad]

That's a ridiculous price, it's a cheap transformer. Source something more local or from someone who knows how to get product into Europe at a reasonable cost (Antek, perhaps?).

Too late, they've processed the order. The only UK 650VCT transformer I could find in EI form was over 200VA. I could have gone toroidal but that was not recommended in the article.

 

[6L6]

If you are competing against that price, have Sowter make one for you, it's going to be cheaper. SOWTER AUDIO TRANSFORMERS

 

[KatieandDad]

I'm hoping that I can cancel as I've just found the same transformer at Mouser for £55 incl delivery.

 

[KatieandDad]

By the way Si. As I'm building the PSU separately, what is the load current that the Impasse presents to the PSU ? It has to be loaded otherwise the 450V caps are right on their limits.

 

[SY]

Figure about 35mA for all stages and both channels.

 

[grimberg]

I tried to purchase the transformer from Allied but it has been out of stock for quite some time. I am thinking about using this one:

Antek - AS-1T300

 

[SY]

It's a little low on voltage, but you should be able to get it to work and get ~390V out of it for the regulator.

 

[KatieandDad]

Mouser have got them - Part No. 546-273AZ.

Allied have them in stock as we speak.

 

[grimberg]

How about this one? It has 2 x 320V and even though it is rated at 50VA, it seems to be enough for the ImPasse.

Antek - AS-05T320

 

[grimberg]

Mouser have got them - Part No. 546-273AZ.

Allied have them in stock as we speak.

I was looking for the one SY mentioned in his article, Allied 6K3VG. That one has been out of stock for at least a couple of months. I am planning to use a HIFI 2000 chassis which is only 3 inches tall. Either model of the Hammond will not fit as it comes from the manufacturer. I would have to use different brackets to mount it horizontally.

 

[KatieandDad]

Have you though about going down the Audio SMPS route. I say Audio with tongue in cheek.

A well filtered SMPS might fit your bill.