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

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PostPosted: 20 Feb 2012, 14:23 
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If the above post is about using a MC cart.with the Groovewatt I would suggest the Le Pacific single jFet MC preamp stage on batteries. Even with a good build you can put it together for about $50. On a close comparison with the more complex more expensive Hiraga MC preamp there is very little in it. Both the Hiraga and Le Pacific are dead quiet and I have had both working with MC carts. which put out 0.3mV. Just match your jFets.

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Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
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PostPosted: 20 Feb 2012, 18:07 
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Hi, A head amp like that is a good solution as well. Very common practice. The transformers are more convenient, but more costly as well. Two half ice cube sized things fit into nearly any preamp. I was quite concerned that they would seriously color the sound, not so. I love the gain without noise bit.

Good listening
Bruce

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PostPosted: 25 Feb 2012, 02:04 
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Hi, I made this amp but used 4 6n2p-ev tubes instead. The bass response is good but the sound is too dark and not so clean. Can these Russian tubes be reason? (causing incorrect RIAA equation due to impedance difference if there is)
I used ECC81 as output tube.


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PostPosted: 25 Feb 2012, 09:59 
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Hi Yes that can be the case. The RIAA network is passive and takes into consideration both the loading of the second stage and the anode resistance of the first. The resistor immediately following the coupling capacitor from the first stage to the network is 89K minus the anode resistance of the first stage. This also is based on the 470K grid resistor of the second stage. Changes to the resistor have a significant effect on the sound. Second, the tube you chose, has a good amplification factor. This is key to minimizing the number of stages, but the values I found for normal cathode currents appear off. It may not function well at the values in the circuit. I suspect that the actual current is quite low for that tube. This will certainly upset the sound.

The sound of the ones using the specified parts are quite neutral and detailed. All the way from 20HZ to well past 20K on the top end ( I just yesterday verified one in my shop). Deviation from RIAA is less than 0.25db the whole way.

Another possibility is how you hooked it into your system. I works fine with nearly any input impedance. I experimented with it using a low output MC pick up with a 20 ohm impedance to a signal generator with over 100K. All pretty much the same. The output though can be effected by extremely low impedances. I recommend 5K or higher although the loss of bass response is quite small all the way down to 1K. Below that it may be noticeable depending on how low the rest of your system goes. Higher values - over 5k are all the same.

Keep us posted on what the outcome is

Good listening
Bruce

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PostPosted: 25 Feb 2012, 16:38 
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It sounds me treble is cut off but I am not sure if the reason is wrong RIAA (possibly yes) or wrong current.I checked the current of 6n2p-ev, each triode consumes around 0.8 mA. B+ is 200 and 215V for the first and second stage. Could you advise if this current is normal for 6n2p-ev? Grid voltage of the second stage is around -1.0 V

I built an I/V amp using 6n2p-ev before, it sounded wonderful. I have some spare tubes, that is the reason why I wanted to try these tubes. I can order some 12Ax7 but we can play with current setup first.

I do not have experience about RIAA equalization, is it possible to re-calculate RIAA network for 6n2p-ev tubes?

My setup is Audio Technica At110E MM cartridge and BEZ T10-B5 805SE power amp. I have tried the system with two line stages, one is with EF86 the other is 5670 SRPP. I also tried it without any line stage and all gave same result.

I built ultra simple 2SK170BL MM amp and the system works perfect with this small amp.

Thanks you for your help.

Oguz


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PostPosted: 25 Feb 2012, 16:43 
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The EF86 was developped specially for phono-stages.

From what I read, it is a great performer, but some people complain about its noise.

I never tried that tube, and I have about 4 vintage Mullards tubes!

Maybe they´ll go to eBay in a near future...


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PostPosted: 25 Feb 2012, 22:38 
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Hi, the current value seems low. I'll see if I can find a decent set of curves for it and maybe it can work. The 12AX7s and clones all work quite well in the 0.5 to 1.0 ma range in this type of circuit. But their max ma is 2.5, the initial data I saw on the tube you were using was 10ma max. So clearly some parameters are off. One thing to do is verify that the components in the RIAA section are correct and hooked up right. I have made mistakes there once or twice because they are all crammed in together. Particularly check the 0.01uf capacitor. It in combination with the resistor (the one mentioned earlier post) form the treble cut.

Still your problem is rather strange, usually the problem is too much treble and not enough bass. The circuit cuts high frequencies by nearly 20 db at 20KHZ and boosts the bass at 20HZ by a similar amount. Other than the tubes did you make any other changes?

Good listening
Bruce

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PostPosted: 26 Feb 2012, 10:10 
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HT Performance wrote:
The EF86 was developped specially for phono-stages.

From what I read, it is a great performer, but some people complain about its noise.

I never tried that tube, and I have about 4 vintage Mullards tubes!

Maybe they´ll go to eBay in a near future...


I also built EF86 phono amp using this circuit:
http://www.keith-snook.info/Schematics/ ... %20PCB.pdf

There was serious hum and Zout was too high, I decided to try Groovewatt instead.
I have a couple of SQ E80F tubes, they give very clean and detailed sound for the line stage. They are absolutely better than EF86 tubes.


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PostPosted: 26 Feb 2012, 10:31 
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Hi Bruce,
I attached a comparison between 12ax7 and 6n2p. I also attached the datasheet of 6n2p-ev.
After checking the RIAA circuit, I am sure everything is fine.
I changed 470k with 220k and treble response significantly increased but sound is so so, still much behind of 2sk170BL MM amp.
You are right, we have two problems:

1. Current of the 6n2p-ev
2. RIAA equalization.

Thank you.

Oguz

Attachment:
12ax7&6n2p_comparison.pdf

Attachment:
6n2p-ev_datasheet.pdf


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PostPosted: 26 Feb 2012, 11:00 
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Hi, Thanks for the data, When I compute it .... the tube is not suitable for this use. There is no combination of values that I can determine that will give you a good result. They will be operating in an unfavorable range and likely to have high distortion. The high gain makes it attractive as a good driver tube. They would probably do well in any of my power amp designs. For preamp use, the current values are very important. Low values keep thermal and shot noise low. This tube can't operate at the low levels involved. Also the Rp would indeed throw off the RIAA network. If you need to get 12AX7s I recommend the ECC83 variants. Particularly the JJ gold pin ones as they are very quiet. Their ECC803S is also very good. I have had poor results with the new production Mullards that are supposed to be quiet. My guess is that the Sovtek 12AX7 might be good based on my experience with it in other circuits. I would avoid expensive NOS or new production tubes, unless they can guarantee they are both quiet and meet the gain specs as well as the anode impedance.

Good listening
Bruce

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