Mono 5W-17W Power Amp using EL84 push-pull - diyAudio
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Old 25th June 2011, 01:02 AM   #1
nbraz is offline nbraz  United Kingdom
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Default Mono 5W-17W Power Amp using EL84 push-pull

Hi everyone,

I'm trying to build a single channel power amp using two EL84 in a push-pull setup, with a switch to select between 5W (triode) and 17W (pentode). The idea started in a discussion with a friend, as he needed a small and portable amp to run his Line6 HD500 (modelling unit). I've always wanted to build a tube amp and so I thought this could be a nice and "simple" project to start with.

As a baseline I'm referring to two designs:
1 - 18W stereo power amp http://www.el34world.com/projects/im...pschematic.gif
2 - Orange Tiny Terror http://www.prowessamplifiers.com/sch...iny_terror.pdf

The Line6 HD500 has balanced XLR outputs (+4dBm 1.23V), which we intend to use with this amp (instead of the usual 1/4'' unbalanced connections). The first question I would like to start with is:

Is there any need for a pre-amp at the input stage in this setup? My friend tried his HD500 with a Hot Rod Deluxe connected to the power amp in (so, no equalization stage) and he complained about the volume being to high (possible clipping). I found some schematics for a similar amp (http://www.blueguitar.org/new/schem/...es_deville.gif) and I've noticed that they use a TL072 op-amp (V1B) at the power amp in stage, which I believe is to bring the voltage up before the phase splitter and also, isolate the internal circuit of the external loop effects. This makes me wonder about the usefulness of a pre-amp stage at the input.

If you think the pre-amp is necessary in this project, what gain would you recommend for the pre-amp and what voltages should I consider at the input of the phase splitter?

Any help with this would be very much appreciated!

Cheers,

Nuno
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Old 25th June 2011, 01:48 AM   #2
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hi, I built this amp and it sounds great, just add the switch to change mode pentode ultra lneal to triode, in ul mode thd is very low at 10W, aprox 0.5%, I hope to serve you, regards
Walter

EL84_Schematic.jpg
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Old 25th June 2011, 02:20 AM   #3
nbraz is offline nbraz  United Kingdom
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Hi Walter,

Thank you for your quick reply! The circuit you shown me looks great!

So, do you reckon I don't need an input pre-amp stage on this circuit?

Thanks,

Nuno
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Old 25th June 2011, 05:18 AM   #4
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Maybe this can help you too:

Mesa Boogie 20/20.pdf
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Old 25th June 2011, 05:53 AM   #5
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Quote:
Originally Posted by walter807 View Post
hi, I built this amp and it sounds great, just add the switch to change mode pentode ultra lneal to triode, in ul mode thd is very low at 10W, aprox 0.5%, I hope to serve you, regards
Walter

Attachment 228684
What is the point in bootstrap there?
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Old 25th June 2011, 11:18 AM   #6
nbraz is offline nbraz  United Kingdom
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Hi Fredenando,

Thanks for the circuit! The details on the voltage values at each stage is quite useful!

Nuno
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Old 26th June 2011, 03:05 AM   #7
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Nuno, the amp built for me no need preamplifier for use with cd player standar, this work well but i dont now output specification of the hd 500 line6, for this reazon looks is another circuit, maybe this is what you want, is more likely to adapt the input signal to your liking, regars

Walter
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File Type: pdf 18w-tmb-schematic.pdf (75.3 KB, 305 views)
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Old 26th June 2011, 07:50 AM   #8
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Nuno-
With all the amp modelling in the Line6Pod, my guess is that a clean-sounding amplifier would work better than adding 'more tube' sound. That said, I'm not sure something very 'hi-fi' is necessary.

Yes, you are correct that you don't need the preamp gain stages of a regular guitar amp if you have >1volt at the input.

I'm not sure about the advantages in using the XLR outs from the Pod (unless going direct into a mixer/board). Also are you planning on tying the R&L channels together at the input?

This amp is for gigging? (18watt amps are really LOUD..)

What sort of speaker are you going to use? (Important).

Cheers
John
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Old 26th June 2011, 11:04 AM   #9
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Quote:
Originally Posted by Wavebourn View Post
What is the point in bootstrap there?
Ultimately, it raises OLG to lower the input drive requirements. Without the PFB included between the PI and input stages, OLG will be reduced, so that maintaining a given level of global NFB will then require a greater input level to produce a given output level. Since this appears to be a commercial design, the PFB was likely included to help meet an input sensitivity specification design point.

Dave
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Old 26th June 2011, 11:24 PM   #10
nbraz is offline nbraz  United Kingdom
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Hi everyone,

Thank you all for your input into this project. Is very much appreciated!

Walter:
Thanks for the schematic. I'm trying to guide myself through specific design rules now, so I can actually understand the differences between amps... I have a rough idea of how the amp is going to look like, and I've been finding the Mullard Valve Amplifier Book quite useful for this. Do you know of any good resources it could be also useful?

John:
Quote:
With all the amp modelling in the Line6Pod, my guess is that a clean-sounding amplifier would work better than adding 'more tube' sound. That said, I'm not sure something very 'hi-fi' is necessary.
I agree with you. My motivation for this project is more as an exercise to learn about tube technology, which I've always wanted. On the end is just a simple power-amp (no equalization stage, etc...), so I'm hoping to get something that sounds reasonably nice and with good sound projection and body, which I found difficult to find in a solid state amp :/

Quote:
Yes, you are correct that you don't need the preamp gain stages of a regular guitar amp if you have >1volt at the input.
Thank you for this One question then, why power-amps like Mesa 20/20 or Marshall EL84-20/20 have pre-amp stages at the input, given that they are used with similar modelling units (e.g. Marshall JMP-1 or Mesa triaxis)? Is it because of the regular 1/4 connections?

Quote:
I'm not sure about the advantages in using the XLR outs from the Pod (unless going direct into a mixer/board).
We have the option for balanced XLR connections, which are mechanically more robust and offer the possibility of reducing noise (created in the wires). We could use a normal connection but since we have the option, why not XLR?

Quote:
Also are you planning on tying the R&L channels together at the input?
My friend (which the amp is for) gives priority to portability, so we're aiming to build just a mono amp.

Quote:
This amp is for gigging? (18watt amps are really LOUD..)
Good Hopefully, if the project goes forward, the amp will be used in gigs and studio.

Quote:
What sort of speaker are you going to use?
Is going to be a 16ohm Celestion Greenback or Vintage 30.

Thanks,

Nuno
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