Suggestions given to a newbie are different (a subset of) possibilities someone with design experience might try. Getting in over your head is easy - but we all start *somehwere*. Someone who has designed in transistors does have a leg up on someone who hasn’t, even if neither never touched a tube in their life.
There are also many reasons to build flea amps. Someone may only want 3 watts, but want the best possible 3 watts. Some are bent on using preamp tubes to drive speakers directly (almost). It may be about power consumption and heat. 6SN7’s are pretty cold compared to 50C5’s. Some only want to limit to 169 volts, for a variety of real valid reasons. Using an OA2 regulator does not top the list, but there are others, like only wanting to spend $20 on a power trafo instead of $150 “because it’s for tubes”. Some are just up to a challenge of using tubes not designed for audio. The goals are different and so are the possible paths.
"Suggestions given to a newbie are different (a subset of) possibilities someone with design experience might try. Getting in over your head is easy - but we all start *somehwere*. Someone who has designed in transistors does have a leg up on someone who hasn’t, even if neither never touched a tube in their life."
As every forum member can see when reading my user CP, I am currently retired. My professional life was an analogue electronics designer of mostly Integrated Circuits for almost 40 years at a company formally known as Philips Semiconductors, now NXP.
But as a student of electronics in the early 60's of the last century also vacuum tubes were then still in use and so real subject of study in my education then.
After my retirement I picked up the tube as active element to design headphone amps. Now I want to make an amplifier producing larger power and a speaker as load. Something which I never have done before. That's why I started this thread.
As every forum member can see when reading my user CP, I am currently retired. My professional life was an analogue electronics designer of mostly Integrated Circuits for almost 40 years at a company formally known as Philips Semiconductors, now NXP.
But as a student of electronics in the early 60's of the last century also vacuum tubes were then still in use and so real subject of study in my education then.
After my retirement I picked up the tube as active element to design headphone amps. Now I want to make an amplifier producing larger power and a speaker as load. Something which I never have done before. That's why I started this thread.
Four 0A2 are available, according to first post: two of them can be connected in series to get 300V. It is a good value for several popular tubes and circuits. For a bedroom amplifier paired with regular speakers, I've found that 1-2W are enough. Due to the low power, transformers are smaller and the build can be very compact. My latest amp is a basic two stage circuit with EL83 as output (triode connected) and EBC81 as gain stage. EBC81 is one half of 6SL7, with a internal shield. Single ended amplifiers such as this one are easy to build: 7 resistors and 3 capacitors per channel. Supply is 320V for the EL83 (300V would be fine) and 250V for the EBC81. If the supply must be 150V, I would use the EL86 tube (noval) or 6Y6 (octal) because they are designed to work with this low B+ value and a low 2,5-3K load, so the output transformer is less critical. Datasheet values are a good starting point for a prototype.
I just dragged my little SE ECL82 amp out to put into my homebrew shed. Driving some Fullrange drivers in backloaded horn this put out plenty enough volume to fill the room, so partnered to a sensitive Fullrange speaker with minimal crossover a single ECL82 is more than enough power to fill a bedroom. This thing uses salvaged UL transformers from old desktop radios which sound much better than they have any right to.
Shoog
Shoog
Yes at "room filling" volume it will sound full range, but at a whisper volume (of a bedroom amp) the bass will simply disappear, the speakers wont be moving much bass. This is why a $69 Amazon Echo Alexa cylinder amp sounds great in the bedroom, they have a small 3 inch subwoofer and profiled loudness curve at the whisper volumes of a bedroom amp, and is omnidirectional.
I think we are totally missing the design challenge behind a tube bedroom amp which is the OP's goal. How to obtain full range and great bass at a whisper volume. A loudness profile maybe? A small sub? A three channel tube design where the third tube channel goes off to a passive sub? Low volume setups that sound full range are difficult. Amazon Echo's have nailed it perfectly, now how do you do that with tubes?
Well, going back to how I listened to music when I was a teenager... I used to leave the Bass control almost cranked, all the time. I'd tweak the Treble control to taste. So... Tone controls!
The active Baxandall tone controls can be done using a 12AT7 or similar high-mu triode with reasonably low rp. MerlinB wrote about this in his hi-fi preamp book. I've modeled it in LTspice and it looks like it should work pretty well.
Only +/-6dB boost/cut, though. Would that be enough?
The active Baxandall tone controls can be done using a 12AT7 or similar high-mu triode with reasonably low rp. MerlinB wrote about this in his hi-fi preamp book. I've modeled it in LTspice and it looks like it should work pretty well.
Only +/-6dB boost/cut, though. Would that be enough?
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As i said a small digital bass amp (20-50W) built into an active sub is the way to go. You can set it to whatever you need to fill in the bottom end. Digital is best for this duty since it can be achieved with minimal size, a valve amp to do the same duty will be a whole project all on its own - and it will do a significantly worse job than the €10.00 digital amp.
Valve bring absolutely nothing to the table when discussing subwoofers.
Shoog
Valve bring absolutely nothing to the table when discussing subwoofers.
Shoog
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This works even better. 3X lower THD, less insertion loss, smoother EQ taper. (Edit: Remove/short out C7. It's no longer necessary.)
Sorry, not meant as a reply to Shoog.
Yes, digital/solid-state would be smaller, more efficient, cheaper, better all-around. I can't disagree with that.
Sorry, not meant as a reply to Shoog.
Yes, digital/solid-state would be smaller, more efficient, cheaper, better all-around. I can't disagree with that.
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Yeah in the end thats the way to go I think, I personally love subwoofers. For a bedroom application a small sub hidden, even easier with a wireless link then you can experiment with placement. Such a system would play full range at a whisper as well as double as a 2.1 system for watching movies in bed! And probably have a very open midrange as you can include a high pass filter switch at maybe 70hz to alleviate the OPT' from any heavy lifting. Of course if adding a HPF you'd have to feed the sub from the input not from the speaker terminals.
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JoeAlders....I was born in early sixties. We grew up listening to the table top radios, small record players and cassette players. None of these systems probably could handle anything lower than 100hz. So we didn't enjoy our music then?
This is ridiculous, from my direct experience I can assure you all you need is a small 1-2 watt amplifier and a set of small and simple Fr or two way book-self speakers, period.
Enough is enough please decide what you want to do and do it right now. We enjoy music or we enjoy our hifi!
Regards
This is ridiculous, from my direct experience I can assure you all you need is a small 1-2 watt amplifier and a set of small and simple Fr or two way book-self speakers, period.
Enough is enough please decide what you want to do and do it right now. We enjoy music or we enjoy our hifi!
Regards
The bass (and high) frequency boost at low volume was solved in the past with coupled volume/tone controls. The attached reference design from Telefunken has been implemented on countless German tabletop tube radios, with good results. And, with some tweaking, it would be possible to meet the OP requirement about the 0A2 (two in series).
Thats an interesting circuit, it uses grid leak bias on the driver which is how I have used the triode in the similar ECL82. This in of itself makes for better bass response.
Shoog
Yes, the 10M grid leak resistor was a given back then, and it works well on ECL86, EABC80 and EBC81 tubes because they were designed and tested to work this way. Chassis must be metallic, because otherwise the grid will easily pick up stray RF from switched mode power supplies, CD player servos etc. On this schematic, I would increase the cathode capacitor value. Loudspeaker impedence is specified as 4 ohm on the schematic: the 110 ohm resistor on the ground side of the volume potentiometer should probably become 56 ohms with modern 8 ohm speakers. The value of the capacitor across the output transformer primary should be chosen experimentally. A standard 2V input should be attenuated 10-20 times to get the tone control working as intended. It is not possible to turn the volume completely to zero. A vintage schematic indeed.
Indeed, during my youth many radios had no more that a few watts output, with output tubes like E443H, AL4, EL41, EL84 etc. and at full power in a living, any conversation was impossible. In the thread of the flea power amp you showed results from the tube cad program. Does that include class AB2?
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