Smash build - 3 small issues

[Stedes]

Hi all

Have just finished my Smash build and could use some help ironing out a few problems:

Amp was built and all resistance and voltage checks as per manual tolerances. Plugged in and working.

Problems:

1. Noticeable hum (probably 50Hz) when turned on through speakers. I've rechecked all connections including earth. Using an earthed cable. Tried different mains cables and plugging it into different wall sockets. Same when plugged in same socket as amp or same socket as source. Hum equal on all 3 inputs and using either output. Hum not changing with volume setting or balance setting.

2. Volume adjustment makes a lot of noise when turned. Almost like crackling but not quite. When turned down there is visible excursion of both woofers on my speakers...as much as say rumble when listening to LPs. Noise equal in both channels and inputs/outputs. Rechecked all connections to volume pot. All look ok.

3. This is perhaps the most concerning to me. There is severe microphony from the chassis plate. I'm getting high pitched sounds through speakers just by barely touching the plate. Best way to describe the sound is the sound you get when you run your fingers down the slats of a car radiator. Any touch to the top plate, even touching one if the control knobs brings it on. Also when listening a few times I get an increasing in volume sound like feedback when you play the electric guitar and turn the guitar towards the speaker. Only way to stop this is to turn the power amp off otherwise even if I pause the music it keeps increasing in volume.

4. Volume adjustment way too sensitive. 1/4 the way up is almost too loud. I suspect this could be the power amp it is connected to. It's a solid state amp with spec as below:
- input sensitivity 2.4V (750W into 8ohms)
- input impedance 100k ohms

I apologise for rambling on but as this is my first build I tried to give as much information about the issues as I could.

As always your help will be greatly appreciated.

Stef

[Paul Birkeland]

Can you describe your amplifier(s) and speakers?  It sounds like you have a system with very high gain, and padding the inputs of your power amp might be a workable solution.  (A solution to the hair trigger volume and hum)

Directly heated tubes are indeed a bit microphonic, and they Smash will benefit from a solid rack to rest on.

-PB

[Stedes]

Hi Paul

Thanks for the quick response.

The Smash is connected to a Musical Fidelity KW750 amp. The speakers are Kudos Audio C20s (small brand). It's a small 2 way floor stander with 90db sensitivity. Source is an Esoteric X-03 CD player (output level 2.5Vrms at 47k ohms). I'm planning to use this with a Seductor eventually and Zu Audio Omens (97db sensitivity).

The Smash is sitting on a dedicated hifi rack spiked to the floor through very thick carpet. I've blutacked the feet to the rack and I've also placed 4 blobs of Blu tack on the corners of the chassis where it sits on the base. This seems to have lessened the effect but it's still there.

I understand microphonic tubes but I can hear ringing over the music when I just run my finger on the volume control! Should it be that bad?

Oh and so that I'm not accused of being all negative can I just say...for $400 this thing REALLY sings!

Thanks for your help.

S

[Paul Birkeland]

Yeah, this isn't all that abnormal when you combine a very powerful amplifier with efficient speakers (even moderately efficient speakers in your case, since your amp is so enormous).

Something like this will help you get by until you change amps:

http://www.parts-express.com/harrison-labs-12-db-rca-line-level-audio-attenuator-pair--266-244

With the amount of gain in the system, the effects of the microphonics will be severely exacerbated. 

-PB

[Stedes]

Thanks again Paul. I'll get some and try.

Lastly...do you think the volume pot is ok to make noise while turning? Or is this just part of the same issue?

[Paul Birkeland]

Yeah, that's also related.  You could sneeze near the preamp and probably hear it through your speakers with that kind of gain.

For reference, the voltage gain of your amplifier is around 30dB, while nearly all of our amplifiers are right around 15dB. 

With those Harrison Labs attenuators in place, we'll have a much better idea of the performance of the circuit in your system.

-PB

[Stedes]

Thanks Paul

Quick search tells me I can get either 10db or 20db attenuators here. The ones you suggested are 12db. I assume 20db would be too much and 10db will be fine?

[Paul Birkeland]

Yeah, 10dB should help enough to get you in the ballpark.

You can also look for some incredibly inefficient speakers if you end up keeping the amp.  Apogee ribbons at ~78dB efficiency would make good use of that monster amp.

-PB

[Stedes]

Paul...

Apogee...been there, done that. Nothing I could afford at the time could drive them well...

Ahh...that Carver amp with the round level meters....

By the way....how hard would it be to put level meters on the Smash or Seductor?

[Paul Birkeland]

Level meters aren't all that tough.  You can drill holes in the wood base and install them there.  The tricky thing that you will need is a meter with high input impedance and its own driver (assuming you want something like a VU meter).

If you found a pair of 0-50mA current meters, you could monitor plate current on each 4P1L, but that wouldn't be super exciting. 

[Stedes]

Yeah, 10dB should help enough to get you in the ballpark.

You can also look for some incredibly inefficient speakers if you end up keeping the amp.  Apogee ribbons at ~78dB efficiency would make good use of that monster amp.

-PB

So...10db attenuation placed between Smash and power amp:

1. Hum is gone
2. Volume knob while still having a tiny bit of audible "woosh" during volume changes (while no music is playing) is now inaudible during music playing
3. Microphonic noises are all but gone while listening. You still get a little bit of ringing if you tap the chassis but in keeping with normal valve ringing
4. Volume control now much more useful. Starting at 7 o clock, normal listening now at 12 or 1 o clock (instead of .

A question: Now anything beyond 8 o clock volume and I'm getting distortion especially around the bottom end. Does this mean I am overloading the inputs? (I seem to remember reading that on another thread about the Smash)

Take care...

[Doc B.]

Yes, the Smash is a little sensitive to being overdriven in the basic form due to the necessary bias arrangement. The upcoming C4S and hybrid shunt regulator upgrade will increase the headroom. I just heard it last week and it's a very nice upgrade.

[Stedes]

Yes, the Smash is a little sensitive to being overdriven in the basic form due to the necessary bias arrangement. The upcoming C4S and hybrid shunt regulator upgrade will increase the headroom. I just heard it last week and it's a very nice upgrade.

You're such a tease...

Will this be ready by the time my Seductor is ready to ship?

[Doc B.]

The upgrade will come out some time after we begin shipping Seductor. There's just four of us here on a daily basis and thus we have to schedule these things out. We had a meeting last week discussing what we want to try to accomplish over the next few months regarding new releases, and a lot of that concerns how we manage our workflow with this small staff.

[Stedes]

The upgrade will come out some time after we begin shipping Seductor. There's just four of us here on a daily basis and thus we have to schedule these things out. We had a meeting last week discussing what we want to try to accomplish over the next few months regarding new releases, and a lot of that concerns how we manage our workflow with this small staff.

I apologise. I wasn't intending to sound like I was complaining. I am just excited by this new hobby that I seem to have picked up and wanted to take advantage of the one shipment to acquire more stuff as Australia is quite far away...
I am finding myself longing for a rack full of wooden boxes now...