DIY Audio Projects Forum
 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 14 Sep 2014, 20:59

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 240 posts ]  Go to page 1, 2, 3, 4, 5 ... 24  Next
Author Message
 Post subject: 6EM7 power amp
PostPosted: 13 Oct 2011, 07:03 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 18:23
Posts: 3773
Location: Australia
This thread started with the posting by a member with his battery powered power amp. This lead to where I suggested a power amp running from a wall wart. But the whole seed of that idea may have blossomed into designing a compact amp with only two tubes. Or maybe a flea power amp (2W) in monoblock form.

The batton fell to Matt to come-up with a simple design using just two tubes. The schematic is posted but in the meantime I have taken a stock of parts and would now like to fashion this 6EM7 amp around the parts on hand. As suggested above this may lead to a pair of low wattage monoblocks powered from the mains. In the meantime take a look at Matt's early design. Simple.

As usual Matt is giving you a heads-up on current and voltage requirements. Not only do you have amp construction concerns but you must plan for the PS.

Attachment:
Schematic6EM7.jpg


You may find something here if you like this tube: http://www.k5dkz.com/6em7.html


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.


_________________
Projects: "Parley" - Tube preamp with Pass B1 buffer | "VoXConcentriC" - Beyma high efficiency 12" speakers | "Black Lace" - all Ozzie tube 6V6 Power amp - old school
Website: [ retro-thermionic ]


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 08:37 
Offline

Joined: 18 Jun 2010, 12:46
Posts: 284
Location: Northern VA
No coupling cap sounds better.
http://www.the-planet.org/6EM7.html


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.



Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 13:33 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 18:23
Posts: 3773
Location: Australia
@rock4016: Thanks for the schematic. Matt what do you think of the above schematic and what is wrong with DC coupling? I have read where it is not always a good thing.

_________________
Projects: "Parley" - Tube preamp with Pass B1 buffer | "VoXConcentriC" - Beyma high efficiency 12" speakers | "Black Lace" - all Ozzie tube 6V6 Power amp - old school
Website: [ retro-thermionic ]


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 13:43 
Offline

Joined: 18 Jun 2010, 12:46
Posts: 284
Location: Northern VA
The problem with Direct Coupling is that you can't roll driver tubes. You'd want to be able to monitor the current in your output tube.


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 16:13 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 18:23
Posts: 3773
Location: Australia
rock4016 wrote:
The problem with Direct Coupling is that you can't roll driver tubes. You'd want to be able to monitor the current in your output tube.

This amp is not about tube rolling. There are others to build if you wanted to roll. Is there any other issues with direct coupling and what if the battery goes flat? Is it charging when the amp is on?

_________________
Projects: "Parley" - Tube preamp with Pass B1 buffer | "VoXConcentriC" - Beyma high efficiency 12" speakers | "Black Lace" - all Ozzie tube 6V6 Power amp - old school
Website: [ retro-thermionic ]


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 18:23 
Offline

Joined: 18 Jun 2010, 12:46
Posts: 284
Location: Northern VA
mwhouston wrote:
rock4016 wrote:
The problem with Direct Coupling is that you can't roll driver tubes. You'd want to be able to monitor the current in your output tube.

This amp is not about tube rolling. There are others to build if you wanted to roll. Is there any other issues with direct coupling and what if the battery goes flat? Is it charging when the amp is on?


The battery gets charged by the tube. I realize the amp is not about tube rolling, but if the tube is replaced for any reason the output current needs to be monitored for the first 15 min or so.


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 19:13 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 18:23
Posts: 3773
Location: Australia
@rock4016: Now I get it. Thanks.

_________________
Projects: "Parley" - Tube preamp with Pass B1 buffer | "VoXConcentriC" - Beyma high efficiency 12" speakers | "Black Lace" - all Ozzie tube 6V6 Power amp - old school
Website: [ retro-thermionic ]


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 19:44 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 873
Location: US Pacific Northwest
mwhouston wrote:
Matt what do you think of the above schematic and what is wrong with DC coupling? I have read where it is not always a good thing.
Before I say anything else, let me say that, in general, I am rather conservative in my design philosophy. This translates to usually allowing experience rather than dogma guide my design choices. As a consequence, I categorically reject any claim that any one topology will always sound better than another. I have always believed that regardless of the topology chosen, good design is what makes an amp sound good. Not the dictates of a certain topology or set of components.

That being said, I have nothing against direct coupling per say, but the topology does impose significant restrictions on the design as a whole. By using the plate voltage of the first stage to provide the output stage upper bias point, then, in general, significant voltage drop has to be inserted into the cathode circuit of the output stage to arrive at the desired operating point. Because the output stage is the high current stage, this generally means that significant power is wasted in the output tube cathode circuit. This translates into a bigger power supply with more required ripple reduction and a build that has to contend with significantly more heat.

This design was supposed to be small and use a back to back transformer PS design. At the chosen output stage operating point (which is near ideal for this triode) the quiescent power lost is about 11.8W. So the total power that the supply has to deliver is on the order of 14.5W (11.8+2.1W/0.8). If we were to use the same operating points (Sec 1: [175v, 1.55mA] and Sec 2: [215v, 45.5mA]) in the direct coupled mode, the supply power requirements increase to approximately 400v @ 45.5mA or 18.2W plus the output, so approximately 21W. And this is all for one channel. The total power supply load goes from ~30W to ~42W; a 40% increase.

The impacts to the build, all to eliminate a single capacitor in the signal path, are I believe, not warranted. Especially when the capacitor is chosen such that the dominant zero is placed outside of the audio band (note the 7.8Hz rolloff frequency) where the impact of the transfer phase characteristic in minimized. There may be good reasons to direct couple, especially in instrumentation amplifiers where near DC response is critical, but I generally wouldn't apply it in an audio amplifier without a very specific reason.

As for the battery bias of the small signal stage, I'm just not a fan. I'm a stickler for choosing my bias points and I don't like the forced effects that a fixed potential imposes (whether battery or diode) including quantized bias potential and no grid drift auto adjustment. I've also found both battery and forward biased diode circuits to have significantly higher noise than good quality modern metal film or wire wound resistors. And seeing as how the noise in the cathode circuit of the input stage significantly affects the noise figure for the entire amp, minimizing noise here is of critical importance.

So these are just my thoughts on the matter. I don't intend to offend anyone's design or design philosophy, it's just that after 35 years of design experience (nearly 25 of it as a professional Electrical Engineer), these are the opinions I've formed. The opinions and practices of others are of course their own and not to be discounted out of hand.

I'll get off my soap box now. :soapbox:

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 20:54 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 18:23
Posts: 3773
Location: Australia
Matt: I had read where a bigger PS is required for direct coupling. The trannies I have (and going monoblock) should be ideal for this project with a little to spare. Thanks for taking the time to explain. We are all a little wiser.

For me this project will be based on your original hand drawn schematic. But I'm ramping it up from the original concept. I intend to make monoblocks and now tube rectification (5Y3GT). I have a whole bunch of Jensens (copper paper and oil), Russian PIO, and Solens to choose from for inter-stage cap and a small 5H choke for each. I have ordered the additional choke (I had one), the 5Y3s, 6EM7s and ceramic sockets. I have to workout how to house the open frame Edcor OPTS but rated at 15W these should help with good bass extension and weight. Should have the parts early next week.

I have wanted to build something like this for a while. Now finally.

_________________
Projects: "Parley" - Tube preamp with Pass B1 buffer | "VoXConcentriC" - Beyma high efficiency 12" speakers | "Black Lace" - all Ozzie tube 6V6 Power amp - old school
Website: [ retro-thermionic ]


Top
 Profile  
 
 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 22:04 
Offline

Joined: 18 Jun 2010, 12:46
Posts: 284
Location: Northern VA
Suncalc wrote:
mwhouston wrote:
Matt what do you think of the above schematic and what is wrong with DC coupling? I have read where it is not always a good thing.

The impacts to the build, all to eliminate a single capacitor in the signal path, are I believe, not warranted. Especially when the capacitor is chosen such that the dominant zero is placed outside of the audio band (note the 7.8Hz rolloff frequency) where the impact of the transfer phase characteristic in minimized. There may be good reasons to direct couple, especially in instrumentation amplifiers where near DC response is critical, but I generally wouldn't apply it in an audio amplifier without a very specific reason.


In effect you are saying that the coupling cap presuming the proper value is chosen, has little or no effect on the sound of the amp.
I couldn't disagree with you more. The coupling cap(if used) in any SE amp will make a huge difference in the overall sound, unless the amp is not designed correctly and has some other problem that hides the effects of various coupling caps.

And it is experience and not Dogma that has taught me that even the very best caps that I can afford (V-Caps) still don't sound as good as no cap at all. Also, most of the amps we use are class A, which by definition are inefficient, so who cares about how many watts the power supply needs to supply? The tubes also get pretty hot, so the added heat from the cathode resistor is not as dramatic as you might think. I've built DC amps with acrylic chassis that after a couple hours of use did not have any heat related problems. I wasn't suggesting that you re design the power supply using your original operating points, I was just offering another option that didn't include a coupling cap.

If I can use a LED or Shotsky diode in the cathode of the first or second stage of a MC phono amp, then they are certainly quiet enough to be used in this amp.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 240 posts ]  Go to page 1, 2, 3, 4, 5 ... 24  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: Google Adsense [Bot] and 4 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Opamp Projects :: DIY Phono Projects :: DIY Cable Projects :: HI-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy