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Old 15th August 2014, 06:46 AM   #1
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Default Aleph as guitar power amplifier

Hi,

I would like to start a new project. Make Aleph amplifier as guitar power amp. I need about 70W / 8 Ohm, 140 / 4 Ohm.

I will need to raise output impedance to 4 Ohms. This will be done via mixed-mode feedback. I will also lower standing bias current a bit to lower dissipation.

Which version of Aleph would be most suitable?

Thank you.
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Old 15th August 2014, 10:19 AM   #2
Zen Mod is offline Zen Mod  Serbia
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introducing mixed mode feedback demands some level of expertise

if you have that , you're able to calc current and voltage demands for intended RMS power

not disputing , just pointing on possible problems
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Old 15th August 2014, 10:36 AM   #3
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The project is propably not good idea, push-pull output stage would be better as it can deliver twice the standing current.
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Old 15th August 2014, 10:46 AM   #4
Zen Mod is offline Zen Mod  Serbia
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you probably know that for guitar amp , power isn't most prominent virtue

better speaker and better tone is worth chasing

instead of power , better to play with Aleph CCS current gain and possible pure SE mode

one channel of Aleph 4 ............. or - other direction - crippled F4 (BA 3 OS too) with slight changes for increased PSU voltage , driven with appropriate (tube ?) gain stage
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Old 15th August 2014, 11:13 AM   #5
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Quote:
Originally Posted by kacernator View Post
The project is propably not good idea
Not at all.
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Old 15th August 2014, 01:08 PM   #6
JMFahey is offline JMFahey  Argentina
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FWIW I copied and pasted this from an Aleph X 100W RMS construction page:
Quote:
How Much Heat Sink?

How much heat sinking do you need for this (or any other Class A) amplifier? LOTS! and then MORE! If your sinks are not HUGE and HEAVY, they're probably too small! A completed amp (high-powered mono, or low-powered stereo) should weigh in near 50-70 lbs. Any less and you've probably skimped somewhere where you shouldn't have. As an aside, Nelson recommends getting at least 15lbs of amplifier for each $1000 you spend on a retail amp... Looks like I've built monoblocks worth about $10k...
So a 140W amp is doable, nobody argues that, it will probably also sound very good, just not very practical.
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Old 15th August 2014, 02:08 PM   #7
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I already have Rocktron Velocity 120, which is all bjt class AB power amplifier for guitar. It makes 120W into 8 Ohms.

I know that Aleph will not be too practical, but I think it should sound HUGE with guitar. Heatsink is not problem, I would use temperature controlled fan, noise is not problem with loud guitar :-).

Interesting is output impedance. Some SS power amps raise output impedance with mixed-mode feedback to about 4 Ohms. BUT some highly regarded SS guitar amplifiers have very low output impedace - Matrix GT800FX and Rocktron Velocity. They propably emulate effect of higher output impedance with some internal EQ tweaking.

Now, do I need to raise output impedance or just use stock Aleph 3 and then possibly tweak?
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Old 15th August 2014, 02:13 PM   #8
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Quote:
Originally Posted by Zen Mod View Post
you probably know that for guitar amp , power isn't most prominent virtue

better speaker and better tone is worth chasing

instead of power , better to play with Aleph CCS current gain and possible pure SE mode

one channel of Aleph 4 ............. or - other direction - crippled F4 (BA 3 OS too) with slight changes for increased PSU voltage , driven with appropriate (tube ?) gain stage
I was also thinking about tube preamp driving F4. Don´t know what would be sonically better. F4 (PP) without any feedback or Aleph (SE) with feedback. I am leaning towards SE.
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Old 15th August 2014, 03:15 PM   #9
Zen Mod is offline Zen Mod  Serbia
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F4 will have gain stage character

so , good guitar pre ........ and you're in game

besides that - source follower stage (F4) is easy and safe for Iq decreasing
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Old 17th August 2014, 12:45 AM   #10
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The Aleph is not a good choice because it must be Class A. I built something like the Aleph that was 50W/4ohm on a CPU heatsink. It runs hot and will probably catch on fire when the fan stops. The F5 Turbo can be setup to transition to Class AB, that will allow you to reduce bias current and use smaller heatsinks. You'll need 40V rails and that means a cascode input stage. You need to be aware that too much input voltage from a tube preamp will kill the input JFETs. Use zener diodes to clamp the input to a safe level.
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