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# A Very Compact Hybrid Amp - Page 26

I have to say that one of things I'm loving about this amp is the fact that it so small. I'd have no problems or worries about removing the tube and throwing the amp in my backpack. Over the last couple of days I've been moving it around from place to place depending on where I'm going to listen, and the portability is outstanding.

I'm also really enjoying rolling tubes - I'm on to trying out a third tube now - an Amprex 8416.

nice tube. it mates well with the beyer premium line.
A few pics of my build..

Need to still drill a hole for the tube on the top plate..

Top toggle switch is for amp turn on and off..lower toggle switch is for heater voltage selection
Nicely done!

Take your time with the tube hole and you'll nail it on center...
To get more output power to drive low-Z headphones and small 8ohm speakers to ~0.25W, would it just be a case of changing the BC327/337 output transistors for larger power transistors (perhaps also a larger walwart)?
nux, Changing the transistors on the output and the walwart isn't the only thing you would have to change. The walwart specified in the design is around 800mA, but in a previous message posted here, runeight suggests that the ouput of the power supply would be around 100mA, which is perfectly fine for use in a headphone amp. If you tried to hook up 8ohm speakers to this amplifier, it would probably only put out about 40mW of power, which is a factor of 6 lower than what you are looking for.

Using Runeight's math in reverse where X is the output current of the power supply for you to get .25 w into an 8ohm speaker : X*X*8/2=.25 watts. X*X=.0625
The square root of .625 = .250A or 250mA, which is 2 1/2 times larger that the typical 100 mA he is suggesting the power supply is capable of.

Start thinking about all the components you are going to have to change and it turns into a different amplfier, in a different form factor.

Sorry for the rant, but I guess I don't get why people want to run speakers with a headphone amp.. Why not buy one of these: ClassT T-AMP TA2024 Audio Amplifier Board SMT 2X15 Watt - eBay (item 230329799289 end time Mar-10-09 18:25:20 PDT)
for \$20, and you could put it in a little box beside your lovely little CTH and have a switchbox on the input to choose which unit you want to listen to. Or put the both together in a bigger box, with a built in switch..
Quote:
 Originally Posted by MrSlim nux, Changing the transistors on the output and the walwart isn't the only thing you would have to change.
Firstly, I realise I may be rude in asking how to modify this somewhat drastically to do another purpose than what it was first intended, especially when it is not fully completed. So I apologies if that is the case, and please let me know if I should be asking this elsewhere.

But I am very keen on making a small tube amplifier capable of driving headphones or a small pair of speakers, and this design looks perfect for that. I realise I will have to modify it a fair bit, and probably use a separate PCB for the output buffer, but thats half the fun (this is DIY after all!).

I gather a 1A 24V regulator is being used, so that should be enough even for 1W output power (with a proper heatsink). The only things holding it back are the BC327/337 on the output buffer and virtual ground sections from what I can see.. Any feedback appreciated. And I didn't take your comments as a rant, I guess I'm just asking if I'm on the right track in how to modify this from a headphone amp to a small general purpose low power TUBE amp
"Why not buy one of these: ClassT T-AMP TA2024 Audio Amplifier Board SMT 2X15 Watt - eBay (item 230329799289 end time Mar-10-09 18:25:20 PDT"

Wow, building my 41hz amp feels like a waste of time now! I am tempted to order one just to see how good it is in comparison.

BTW Nux, MrSlim is right. I had a similar idea to you but realized that it wasn't worth it in the end after pricing out the difference in required transformers etc.. You could figure out how to use the HP amp as a preamp though and run it through a class t amp like the one above. The S.E.X. amp from Bottlehead does what you want. Transformers are what can really drive the costs on such projects. It is too bad they cost so much because I would have built multiple guitar amps by now otherwise.
mOb1liz3,
I'm part way through building a Amp6 myself.. the board is done, just need to get a heatsink and case it up. That was my thought exactly though, it sure looks like an interesting idea. Only, they didn't exist a year ago when I bought the stuff originally (or longer.. shesh) . I saved a thread somewhere on doing Mod's to it also..

Edit: There is a Blog of one person's work here: http://finetone.blogspot.com/2008/10...r-further.html

Edit2: The thread on the board: http://www.diyaudio.com/forums/showt...5&pagenumber=1
Quote:
 Originally Posted by TimJo < snip > I'm also really enjoying rolling tubes - I'm on to trying out a third tube now - an [Amperex] 8416.
When a way for others to build this amp comes about, perhaps as soon as month's end, think a dedicated tube thread for this amp would be helpful. Since it seems that production PCB/BoM will not deviate electronically from about mid-proto process, tube impressions will not differ between proto/prod.

There will be various synergies (e.g. tubes w/given cans), house sounds, etc. I'm finding my Amperex BB 6dj8 *very* similar to the \$7.50 USD ebay std Amperex 6dj8 white-label I've mentioned (Edit: Both of which I love even tho D2000s already have plenty bass). And I wonder if their 8416s, 12au7s, etc are notably different sounding.

Quote:
 Originally Posted by m0b1liz3 "Why not buy one of these: ClassT T-AMP TA2024 Audio Amplifier Board SMT 2X15 Watt - eBay (item 230329799289 end time Mar-10-09 18:25:20 PDT" .... Wow, building my 41hz amp feels like a waste of time now! I am tempted to order one just to see how good it is in comparison.
Ha ha... I'm thinking the same & I need another speaker amp like a hole in the head. Better for another thread but perhaps using this headamp w/some source switching, etc. as a pre is not that far off base... It does have muting delay + offset protection.
Also better for another thread, but maybe this or this are t-amp mod threads MrSlim was thinking of...
You know, I shouldn't even mention this, but it would not be hard to make a speaker amp version of this amp. It would be quite a bit bigger and a 1A wall wart would be absolute minimum. There would be more heat burned inside the box and a serious regulator heatsink would be necessary.

But the rest could be done without much difficulty on a bigger board.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
Ha, now the Cat is out of the Bag..
Quote:
 Originally Posted by MrSlim Ha, now the Cat is out of the Bag..
Maybe Cats over time... The balanced itch could get scratched 1st, who knows? First things first tho.
how about a prototype of that once this has settled down..I would be willing to sign up for one of those boards if at all you guys go ahead with it...
Quote:
 Originally Posted by cfcubed When a way for others to build this amp comes about, perhaps as soon as month's end, think a dedicated tube thread for this amp would be helpful. Since it seems that production PCB/BoM will not deviate electronically from about mid-proto process, tube impressions will not differ between proto/prod. There will be various synergies (e.g. tubes w/given cans), house sounds, etc. I'm finding my Amperex BB 6dj8 *very* similar to the \$7.50 USD ebay std Amperex 6dj8 white-label I've mentioned (Edit: Both of which I love even tho D2000s already have plenty bass). And I wonder if their 8416s, 12au7s, etc are notably different sounding.
Agreed. Besides the portability (which is what really makes this amp shine in my book) the tube rolling is turning out to be a lot of fun. This morning I am trying out an RCA 6189 (black plates, triple mica, square getter) manufactured in '58. I'm liking what I am hearing so far, but only an hour out of the box after sitting around for fifty years, I know it has yet to reveal itself. It's like letting a genie out of the lamp...
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