A cure for the lock down blues - a Bel Canto SETI40

[jahsavage]

Our Xmas was cancelled like everyone else's so feeling a little down I made an impulse buy on ebay to cheer myself up. I purchased a Bel Canto SETI40 amplifier. Its a single ended amplifier with zero feedback and just one capacitor in the signal path. I noticed it had very good reviews but at the time did not realize it was 21 years old.

The amp arrived just before Brexit but unfortunately it was not well packed. One of the 845 tubes was broken and the other made worrying noises when I tapped it. The package came with a spare set of cheap Chinese tubes but only one of these worked. I also read online that one of the output transformer had been replaced.

Not to worry I ordered a set of Psvane Acme 845 from China, but after 4 weeks I heard nothing. After a few email exchanges I was beginning to doubt they ever would. A couple of weeks later I then heard about the terrible quality issues at Psvane and the high percentage of DOA and the dreaded red plate issue. At this point I decided to cancel the this order and order some Linlai tubes from another dealer. The Psvane supplier claimed they had sent the tubes, to my surprise the Acme 845s arrived a week later and another week on a set of Linlai E-845!

As you might imagine I was some what concerned when it came to switch on but to my delight not only did the tubes work they did not suffer from the dreaded 'red plate' issue. I thought I already had some very good amplifiers but this amp with the Acme tubes sounds sublime. The bass is rich and absolutely immense which for a jazz and a reggae fan like myself is ideal.

[Puffin]

Looks like you dodged and bullet there. Don't think I have seen one of these before. I luv a good dose of Dub, Bill Laswell, Jah Wobble, Augustus Pablo, Sly & Robbie and many others.

[jahsavage]

Looks like you dodged and bullet there. Don't think I have seen one of these before. I luv a good dose of Dub, Bill Laswell, Jah Wobble, Augustus Pablo, Sly & Robbie and many others.

The Suite by Public Image was sound system like. There was so much power in the bass I had to take 0.3mh off my cross over to reduce the sub-sonics.

[jahsavage]

The mid range sounds very strange via this clip but you'll get the picture as regards the bass sound.

I'm stil in the process of refining the speaker cross overs to get the best from the amp, hope you enjoy :-

[jahsavage]

After playing around with the cross-overs I found reducing the inductor from 1.8mh to 1.5mh certainly helped but some for music such as Natty Dread by Bob Marley the bass was still too strong. Reducing the inductor further did reduce the bass but the music started to loose its magic. I tried adding resistors between the inductor and speaker, this worked very well except I felt some music lacked bass, having to change resistor value based upon the music was not idea.

Since I had a far bit of acoustic foam in the garage I thought I would try lining the interior, to my delight this tamed the bass and improved the mid range at perhaps a small loss in realism. I shall try removing some foam to try and find the optimum covering.

I also have some Mundorf CFC10 inductors and some more Duelund Oil capacitors on the way so the wife going to have to put up with wires in the lounge for a few more weeks.

[jahsavage]

After adding and removing acoustic foam tiles over the period of a week, I determined a great or average sound was determine by the presence or absence of half a 30cm sized foam tile!

Speaker design is a real crazy art.

[Macca]

It's not for the faint hearted is it?

I like where you've gone with those speakers though.

[jahsavage]

I often wonder why more people do not modify their speakers, by playing around you can match you speakers to your room and amplifier just by removing and adding acoustic foam (eBay £12).

Once you start to get curious you might want to consider upgrading the cross overs. I'm fortunate in that my speakers have a very simple crossover which is reasonably accessible.

the original crossover

my transcription of the circuit diagram

My first step was to update all the components with quality ones from Mundolf and Dueland. This may have been too bold a first move, since although the sound was going to places the original crossover did not, things were not quite right.
The bass was over powering so I decided to reduce the inductor size by removing a few turns until the inductor matched the weight of a Mundorf 3.2mh. This was a step in the right direction but the bass was still too strong.
After about 10 months of fiddling and buying some smaller inductors I settled upon 1.5mH which certainly makes me question my original circuit diagram.

latest crossover

While experimenting with roll off capacitance it quickly become clear that 40uf of Mundorf and 4.4uf of Dueland Oil Capacitors offered something special, by adding various small values between 0.22 and 0.68uf I found could make some music really shine but I discovered this improvement varied dependent upon the music.
For this this reason I settled on a four way switchable crossover so I can tweak the crossover as required.

For anyone interested the difference between the Mundorf CF10's and CF12's is on initial impression quite small but after a while you realize that some magic X ingredient has been added to the sound making it more life like and enjoyable.

[jahsavage]

As regards the Bel Canto SETI40 I've also been busy, I noticed the circuit diagram I had showed one bypass capacitor of 0.22uf, mine probably from a slightly earlier version had a 0.16uf Bel Canto special. I replaced the 22 years young capacitor with a 0.22uf Audyn Copper Max.
This was a significant improvement to the point that I thought I'd might as go for it with a VCap CuTF.



circuit board back

While I was fiddling I also fitted a new a pair of gold plated 845v sockets since the old one looked a bit manky.



While looking at the circuit diagram I noticed the remote volume control uses a Cirrus Logic CS3310 DIP16 chip for its resistor tree and control. In 20 years I hoped they may have been an upgrade, although the DIP16 package is more or less obsolete I discovered the chip is still being made today.
After searching the web I came up with the Texas Instruments PGA2311PG4 which was produced in 2016 to a much higher spec. Fortunately eBay came to the rescue here and I've ordered a couple for £25. I'll let you know if it works and if I can hear any difference.

I also fitted in two matched sets of Psvane's 12AX7-T-MII/2 which provided a larger sound stage than the existing JJ's which were of unknown parentage.

bel canto gold

PS/ I must get around to adjusting the 845 sockets so the tubes stand straight!

[jahsavage]

The Bel Canto is a 845 SE design with a remote control! I've updated all the tubes and the single internal capacitor but I'm aware of the bad press digital volume controls get so I wondered if I could improve things?

The amp uses a Cirrus Logic CS3310 and after a bit of internet research I discovered Texas Instruments make a pin for pin replacement that was last updated in 2016. I wondered what sort of improvement almost 20 years of semiconductor development could bring to the design?



Here's a comparison of the Cs3310 and TI PGA2311 A chips :-

THD was 0.001 % now 0.0002%
Dynamic range was 116db now 120db
Channel cross talk was -110db now -130db

The TI chip is available from Mouser Electronics for £8.50 so I think its worth a punt.