Spud + DAB module: what valve & PS?

[tristanc]

As a winter project I'm looking to build a simple, single channel amp - used to amplify the output of a DAB module. Basically replacing the Pure DAB radio in the kitchen.

Keen to get opinions from the experts here on valve & transformer choice.

Some design thoughts:

- valve(s) should be obtainable in UK / EU (I'm scarred trying to get 6JN6s for my DCPP...)
- will drive a small (4-6in) speaker so fidelity isn't, in reality, a concern (but it's still nice to tinker and make it 'good' up to the speaker)
- DAB module requires 6-24V DC (150mA @ 12V DC)
- keep things small (ie, suitable for the kitchen)

I had thought a PCL82 or PCL86 as I could use the same DC supply for heaters and DAB. Basically a noval spud.

As for power supplies, initially, after searching around, I thought I could use a laptop PSU to give 19V @ 3A in to a cheap eBay SMPS for B+. I could also use the same 19V via a buck convertor for the heaters & DAB. Alas, I see this may be an awful thing to do noise/interference-wise.

So, nixing that idea, how would one easily generate a suitable B+ and the necessary DC? And for what valve?

I've had a go at knocking together a schematic below based on previous designs referenced here. This assumes a SMPS and a PCL86. I've added a CCS lifted from Merlin's book for the driver triode (just because I'd like to try it) and put a placeholder for OT to driver cathode NFB. I need to review the power supply situation.

I'll look in to getting a small PCB made and, if things go well, make a stereo version for the office!

 

[12E1]

You should avoid any laptop or SMPS supplies for a DAB receiver unless you know that it's output is pretty clean at DAB frequencies and that it doesn't radiate significantly. DAB radio signal levels are relatively low, and although the system is pretty robust, if the signal gets marginal, reception will fall off a cliff. Also, make sure you have a decent ground-plane for the antenna unless using a proper external aerial.

You may not feel competent, but that's a pretty expensive DAB solution, and it uses one of the cheaper DAB modules. You may do as well or better by buying a cheap DAB radio and stripping it down.

 

[Eli Duttman]

For the B+ PSU, "full wave" voltage double the O/P of a Triad N-68X isolation trafo. UF4007s are more than good enough, in this application.

I'm uploading a HIFI version of a 6BM8/ECL82 setup that's based on a Mullard original. A tip of the hat to Jeff Yourison, who turned my "hen scratches" into a proper drawing.

 

[PRR]

...I've had a go at knocking together a schematic below ...

You are taking NFB, from an un-grounded speaker, to an LED. Surely not what you want.

 

[Shoog]

PCL82 is an excellent choice, cheap and readily available and great sounding.
I have had success with similar DC-DC converters for a B+ supply, I had no real issues with noise and it was more than capable of driving a nice big filter cap (200uf), add a choke for good measure.

Shoog

 

[316a]

You may not feel competent, but that's a pretty expensive DAB solution, and it uses one of the cheaper DAB modules. You may do as well or better by buying a cheap DAB radio and stripping it down.

Or just buying a DAB radio and using it as-is

I would personally look at a cheap single board computer such as Raspberry Pi and maybe add an I2S DAC to it . Most radio stations can be streamed along with any stored music files via USB or NAS drive . Much more flexible...

316a

 

[Shoog]

Or just buying a DAB radio and using it as-is

I would personally look at a cheap single board computer such as Raspberry Pi and maybe add an I2S DAC to it . Most radio stations can be streamed along with any stored music files via USB or NAS drive . Much more flexible...

316a

PiZeroW would do fine. I use one for an intelligent LED projector running Kodi. A bit sluggish on the controls but otherwise perfectly adequate, with the new built in WIFI its perfect.
The only fly in the ointment is that if your internet is poor quality then radio streamers tend to drop the connection rather often.

Shoog

 

[tristanc]

Thanks all.

I know keeping my existing Pure DAB radio would be simpler, but where's the fun in that? Also, a streaming system would have been great too - until the BBC switched off their services in favour of their proprietary tech...

So I need a new revision with a few tweaks:

- Ditch the SMPS, although it would be nice, as it may mess up the DAB signal
- Go standard rectification & CRCR flltering for B+ (voltage doubled or not depend on PT)
- Ground the speaker properly
- On the feedback - is it not possible to connect the fed back signal to the cathode with LED bias?

For the digital power supply, I'm thinking a Mean Well IRM-03-12 to provide 12VDC. This can be dead-bugged or I can put it on a PCB. IRM-03-MEAN WELL Switching Power Supply Manufacturer

This then opens me to using the ECL82 or ECL86 (higher cost but still OK) and thus standard transformers (ie cheaper).

OR, I can get a transformer with, say, a 13V heater tap, rectify this to supply DC to the digital power supply and potentially DC to heaters, although I could do AC for even simpler. Something like: TSTA 0050/002 - Mains transformer for tubes - Shop Toroidy.pl

This allows the PCL82 or PCL86.

Then a cheap and cheerful 5k OPT (eBay) and a little speaker will get me there.

I am looking at putting the electronics in a small Hammond enclosure, which is itself put inside a laser-cut plywood box (to be designed).

 

[Fenris]

Thanks all.
- On the feedback - is it not possible to connect the fed back signal to the cathode with LED bias?

Look at the LED in AC terms. If you look at the datasheet for an "average" LED and look for the change in voltage with change in current, it works out to something between 3 and 12 ohms "equivalent" impedance. It's going to require a lot more drive for feedback (and grounding of the speaker to give a reference point). Is global feedback even necessary? Local feedback may be sufficient, especially if using the output pentode as a triode.

 

[stocktrader200]

use a 10 or 20k volume pot, There is no reason to use 1m as all non tube outputs can drive 600 ohms effectively.

 

[tristanc]

Look at the LED in AC terms. If you look at the datasheet for an "average" LED and look for the change in voltage with change in current, it works out to something between 3 and 12 ohms "equivalent" impedance. It's going to require a lot more drive for feedback (and grounding of the speaker to give a reference point).

Interesting - I did wonder why I didn’t see it used... So the 3-12Ohms is the bottom R of the voltage divider, so basically of the order of the speaker Z? Using a standard R_k / C_k instead means a, e.g., 2.2k R at the bottom of the divider.

Is global feedback even necessary? Local feedback may be sufficient, especially if using the output pentode as a triode.

That’s a good point - I was copying others... People also mention plate to plate. When I breadboard I’ll test a couple of options as best I can. Will be fun working through how to optimise it and what difference it makes.

use a 10 or 20k volume pot

Good catch - thanks. A hangover from a guitar amp schematic...

I’ve been doing more searching and this post from 12 years ago led to some good schematics, particularly the power supply: ECL82 (6BM8) PP Stereo

I need to source a suitable PT, something like pri 240:sec 240 (0.1A) & 18 or 15V (1A) with the latter regulated to 12V. Or sec 240 & 12.6 with the 12.6 applied directly to the heaters and a DC supply sourced elsewhere.

 

[12E1]

Thanks all.

I know keeping my existing Pure DAB radio would be simpler, but where's the fun in that? Also, a streaming system would have been great too - until the BBC switched off their services in favour of their proprietary tech...

So I need a new revision with a few tweaks:

- Ditch the SMPS, although it would be nice, as it may mess up the DAB signal
- Go standard rectification & CRCR flltering for B+ (voltage doubled or not depend on PT)
- Ground the speaker properly
- On the feedback - is it not possible to connect the fed back signal to the cathode with LED bias?

For the digital power supply, I'm thinking a Mean Well IRM-03-12 to provide 12VDC. This can be dead-bugged or I can put it on a PCB. IRM-03-MEAN WELL Switching Power Supply Manufacturer

This then opens me to using the ECL82 or ECL86 (higher cost but still OK) and thus standard transformers (ie cheaper).

OR, I can get a transformer with, say, a 13V heater tap, rectify this to supply DC to the digital power supply and potentially DC to heaters, although I could do AC for even simpler. Something like: TSTA 0050/002 - Mains transformer for tubes - Shop Toroidy.pl

This allows the PCL82 or PCL86.

Then a cheap and cheerful 5k OPT (eBay) and a little speaker will get me there.

I am looking at putting the electronics in a small Hammond enclosure, which is itself put inside a laser-cut plywood box (to be designed).

In their defence, for internet radio devices the BBC currently offer MP3 using Shoutcast, AAC over DASH, and AAC over HLS. Not particularly proprietary, but potentially subject to change from time to time. Some good info here...
http://iplayerhelp.external.bbc.co.uk/radio/other/streaming_codecs_bitrates

For yor 12v power rail, I'd avoid that small SMPS - it may radiate or polute the inout rail or output rail. I like the idea of simple regulation from a 13VAC that is used by the heaters.

 

[tristanc]

In their defence, for internet radio devices the BBC currently offer MP3 using Shoutcast, AAC over DASH, and AAC over HLS. Not particularly proprietary, but potentially subject to change from time to time. Some good info here...
http://iplayerhelp.external.bbc.co.uk/radio/other/streaming_codecs_bitrates

Ahh! I didn't know they reversed their decision. I used to have my Pi set up to stream, worked really well until they removed the direct URLs... BBC Streaming Radio Script for MPD – Tristan Collins

There's another weekend project!

 

[12E1]

Ahh! I didn't know they reversed their decision. I used to have my Pi set up to stream, worked really well until they removed the direct URLs... BBC Streaming Radio Script for MPD – Tristan Collins

There's another weekend project!

PM sent for to help you with that project...