Class-D and Tube buffer/preamp off same 12V SLA? - diyAudio
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Old 30th April 2015, 04:01 AM   #1
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Default Class-D and Tube buffer/preamp off same 12V SLA?

Hey everyone, I don't mean to overpopulate the site with similar posts but I wasn't able to find a clean answer to this so I will try asking a few more specific questions!

I am building the common Ammo Can Boombox and would like to decide on an amp setup. For now I plan to run a pair of FaitalPRO 4FE35's (91dB/1w each) powered by a 12V 7AH SLA battery.

I plan to run this Class-D amp, it seems very appealing!

Stereo 20W Class D Audio Amplifier - MAX9744 ID: 1752 - $19.95 : Adafruit Industries, Unique & fun DIY electronics and kits

HOWEVER, I think it would be super cool to incorporate a Tube into this build! (keeping with the era) There are a few options I see if I plan to do this:

1) put together a circuit as seen in this video: https://www.youtube.com/watch?v=BTrUJC_vpyc (I believe he said something about changing the bias to allow the Tube to run at such low voltage)

2) Acquire a 12VDC to 12VAC inverter (if such a thing exists, haven't found much about them.) once 12VAC is present, run this little guy in front of the class-amp. Yuan-Jing 6N3 Tube Pre-amplifier Board The limited reviews look good so far!

Now for the questions:

-IS there a good way to get from 12VDC to 12VAC? compact required, It's going into an ammo can! that would allow me to run a properly configured, proven tube preamp that apparently does a pretty good job of adding that loved tube distortion.

-If I don't use the premade preamp and opt to run my own circuit based on the one in the video, will I get any of the "Tube sound" with such a low plate voltage? I read in another post that the 12AX7 like high voltage but the 12AU7 or even a 6volt tube would work better.

THANK YOU for the input! I am still very much so a beginner at tubes so I try to learn more each day!
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Old 30th April 2015, 05:28 AM   #2
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Quote:
S there a good way to get from 12VDC to 12VAC?
You might not even need to change it to 12VAC, as it will be rectified back to DC. With most tube preamp, there are two power supplies needed: one for heaters (which can be AC or DC) and one for tube plate (B+, has to be DC).

I notice that there is 7805 regulator on the Yuanjing board. This might be used to supply the heaters. The remaining question is the tube plate supply. It can simply be a rectified 12VAC which yields something as low as 12-15VDC (in which case your SLA battery works just fine) OR it can go to a voltage multiplier first to get something higher than 15VDC (in which case, your SLA battery doesn't work). Without any schematic, we can only guess what is the plate supply used on the Yuanjing preamp.

Fret not! Ebay is loaded with high voltage boost converters that can boost your 12VDC to sufficient voltage for a 6N3 based tube preamp.

So.. if you want to buy a kit, option 2 is good.

If i were you however, i would build this Unity Gain Harmonic Restorer
A New Look at Harmonic Restoration
160VDC can be obtained by using Nixie voltage converter (many on Ebay).

Of course, do you really want something as inefficient as tube to reduce the playtime on your SLA battery significantly?
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Old 30th April 2015, 08:24 AM   #3
DF96 is offline DF96  England
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If you want "tube sound" then a good way to achieve this is to use a rather low supply rail voltage. 12V will do. This ensures that the valve is poorly biased so it will generate the distortion you seek.

If you want good sound reproduction with tubes then you need 200-300V on the supply rail and then you can have very low distortion.
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Old 30th April 2015, 08:38 AM   #4
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If you require a valve sound, use a FET. then by all means light up the valve heaters for show.
FETs behave similarly to valves if over driven.
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Old 30th April 2015, 02:52 PM   #5
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Considering the great variety of SS electronics found in cars these days, I suspect that finding circuitry to go from "12" VDC, out of a lead/acid storage battery to +/- 25 VDC bipolar should not be too difficult. Once you have that bipolar PSU, setting up a highly linear, CCS loaded, cathode follower using the 6GM8/ECC86 is easy enough.
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Old 30th April 2015, 03:42 PM   #6
Koifarm is offline Koifarm  Netherlands
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As Eli said use ecc86 as cathode follower with 12v anode supply. I did that several times.
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Old 30th April 2015, 09:50 PM   #7
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Quote:
Originally Posted by JonSnell Electronic View Post
If you require a valve sound, use a FET. then by all means light up the valve heaters for show.
FETs behave similarly to valves if over driven.
Now it seems that all we need to do is get some tube bodies filled with some FET circuitry and an orange LED =)

I had looked into FET's for a little bit this morning, it looks like their used often in tube emulator circuits.

Hmm... the ECC86 looks like a good idea! once I learn how to implement a cathode follower.

Will that ECC86 offer any distortion/coloration with that low voltage?

Also, would you recommend any place to start looking for schematics? The 12V anode supply is one thing but are the heaters also wired to 12v?
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Old 1st May 2015, 09:32 AM   #8
DF96 is offline DF96  England
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An ECC86 will offer less distortion at low voltage than most other valves, because it is designed to run at low voltage. I am unclear whether you require distortion or want to avoid it.
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Old 1st May 2015, 10:41 AM   #9
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Quote:
Originally Posted by chips and palsa View Post
Now it seems that all we need to do is get some tube bodies filled with some FET circuitry and an orange LED =)

I had looked into FET's for a little bit this morning, it looks like their used often in tube emulator circuits.

Hmm... the ECC86 looks like a good idea! once I learn how to implement a cathode follower.

Will that ECC86 offer any distortion/coloration with that low voltage?

Also, would you recommend any place to start looking for schematics? The 12V anode supply is one thing but are the heaters also wired to 12v?
The heaters can be wired in series to operate on 12V.
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Old 1st May 2015, 06:42 PM   #10
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Default Class-D and Tube buffer/preamp off same 12V SLA?

Quote:
Originally Posted by DF96 View Post
An ECC86 will offer less distortion at low voltage than most other valves, because it is designed to run at low voltage. I am unclear whether you require distortion or want to avoid it.

Well I may have this wrong but does the tube "warmth" and "richness" come from distortion in the form of low order harmonics or is it possible to get that added sound benefit from a low voltage low distortion tube? I guess if the distortion that you're speaking of is the same that the preamps give in a JCM900 then I should avoid it!

I just want the usual benefits of running a tube preamp with a solid state (and also that subtle glow, I cannot deny that)

The yuan jing preamp has a potentiometer who's soul purpose is to dial in the "warmth" and a lot of people are very pleased with the detail, possibly in the form of pleasant coloration it added. That's all I'm looking for! Is that still possible with the ECC86 cathode flower?


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