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Old 2nd October 2010, 07:18 PM   #1
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Default KT88 hybrid amplifier

Hello,

I am an electronic engineering technologist student, and for my capstone project I am building a stereo amplifier using a solid state front end and a tube power amplifier. One of the requirements for the amplifier is that an ARM microprocessor be used somehow. I have decided to use the microprocessor to control the volume, bass/treble control, speaker balance and input select.

I have always been facinated with vacuum tubes and thought it would be a good idea to incorporate it into my ampliifer. The tubes I have chosen are KT88's (I bought 4 matched Electro-Harmonix tubes) in a class AB configuration. I want the power output to be 50-75 watt per channel with THD <%1. Since I have never built a tube amplifier before I would like some suggestions from you guys.

One of my main concerns is combining a solid state pre-amplifier with a tube amplifier. I can't find a lot of information on this setup, so any comments or information sources would be greatly appreciated.

Is there anyone who has experience with implementing digital potentiometers in an amplifier? They seem pretty straight forward. Also, could there be noise issues with running an analog signal through a digital pot?

Thanks
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Old 2nd October 2010, 07:35 PM   #2
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Sorry I have no experience with digital pots (the concept causes uncontrolled twitching in my right eye for some reason ) but I have a couple of thoughts for you. Since you are going to have a micro processor in there maybe it could be used to mange the output tube bias as well.

Another possibility is to use motorized pots (or step attenuators) with the motors or relays controlled by the micro processor.

If you are going SS front end I would resist the temptation to use high gain high FB devices like op amps. Are you doing phono preamp also or just line amp in the front end? Simple discrete jFET front end would seem to be most in keeping with the output stage you are planning. I can't help with the details but it seems that I have read here some posts on using local FB on FETs to get very triode like characteristics.
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Old 2nd October 2010, 07:59 PM   #3
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Hmmm...THD < 1% - why? As long as the solid state section behaves and only the tubes produce harmonics you can allow much more than that.
You mainly get k2, one octave above the signal, and that fits in very nicely.

Clobbering tubes into submission by using too much feedback (word?) makes them sound as lively as old socks.
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Old 2nd October 2010, 08:03 PM   #4
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Something like this might be useful:

DS1666 5V Pseudo Log Audio Digital Potentiometer - Overview
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Old 2nd October 2010, 08:05 PM   #5
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P.S. As to the digital pot - you might contact Ed Schilling at
the horn shoppe

Tell him IŽll send Helen a kiss and three hugs if he shares a bit of electronic sorcery with you.


Pit

(DonŽt phone him when you are pressed for time - that man doesnŽt believe in two-minutes-info)

Last edited by Pit Hinder; 2nd October 2010 at 08:10 PM.
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Old 2nd October 2010, 08:52 PM   #6
cjkpkg is offline cjkpkg  United States
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I would recommend, say, a Pass front end of some kind...maybe a JFETBOZ or something...couple that output to your KT88 power stage and whatever add-ons you want and you will have something that sounds very nice.

I was using my JFETBOZ as preamp for my simpleSE using KT88s and it sounded very nice. I would imagine in PP configuration the power would be ample...
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Old 2nd October 2010, 10:08 PM   #7
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Good suggestions have been made. Do what's necessary to get a modest gain "sand" front end that's linear. Then, you will have enough Volts to use a 12AT7 based differential splitter/driver, like that found in "El Cheapo" (schematic attached).

UltraLinear mode "finals" will get you the power you desire. Enough loop NFB is present to provide a decent damping factor and a THD under 1%.

The 'T7 sounds good with 200 to 220 V. on the plate and an IB of 3 mA.

Edcor's model CXPP100-MS-3.3K O/P trafo will give good results, without making you destitute.
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File Type: gif ElCheapo-23jun06-map.gif (36.3 KB, 532 views)
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Old 3rd October 2010, 02:34 AM   #8
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Quote:
One of the requirements for the amplifier is that an ARM microprocessor be used somehow.
I have put chips and tubes together several times. About 15 years ago I built a guitar amp using a SS front end and tubes for output. All of it was controlled by a PIC processor. At the time National Semiconductor made a stereo preamp chip which had the volume, bass, treble, and balance functions controllable via SPI. They also made an equalizer chip. I used some Panasonic rotary encoders, and LED readouts for a cool amp.

More recently I used a dsPIC chip to do active power supply modulation (Class H operation) on a vacuum tube amplifier. It was entered into a design contest sponsored by Circuit Cellar magazine where it won a prize, and lead to me writing a magazine article about it.

Why am I telling you this? Well it seems that Circuit Cellar and Elektor are running another contest. They want the world to learn about NXP's (formerly Philips) new MBED board. First there was the Basic Stamp, then came Arduino. Now there is MBED. MBED is a tiny board resembling the DIP version of the Basic Stamp. It contains an ARM Cortex M3 chip with plenty of processing power. The new unique twist here is that you program it via the web. THe compiler resides in the "cloud." If you fill out a form on the contest page they will send you one of the boards absolutely FREE. I got mine last week. If someone likes your entry, you could win cash. If you write an article about it you could get a job offer, or even a phone call from a CEO.

NXP mbed Design Challenge

Yes, I'm going to enter something. It will have tubes and chips, but it won't be a stereo amp.
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Old 21st October 2010, 05:42 PM   #9
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Thanks everyone for the informative replies. I definately will consider everyone's opinion. I am sure I will have plenty of more questions as the project moves forward. I'll keep you posted as I make progress.
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Old 24th October 2010, 03:35 PM   #10
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Quote:
Originally Posted by tubelab.com View Post
I have put chips and tubes together several times. About 15 years ago I built a guitar amp using a SS front end and tubes for output. All of it was controlled by a PIC processor. At the time National Semiconductor made a stereo preamp chip which had the volume, bass, treble, and balance functions controllable via SPI. They also made an equalizer chip. I used some Panasonic rotary encoders, and LED readouts for a cool amp.

More recently I used a dsPIC chip to do active power supply modulation (Class H operation) on a vacuum tube amplifier. It was entered into a design contest sponsored by Circuit Cellar magazine where it won a prize, and lead to me writing a magazine article about it.

Why am I telling you this? Well it seems that Circuit Cellar and Elektor are running another contest. They want the world to learn about NXP's (formerly Philips) new MBED board. First there was the Basic Stamp, then came Arduino. Now there is MBED. MBED is a tiny board resembling the DIP version of the Basic Stamp. It contains an ARM Cortex M3 chip with plenty of processing power. The new unique twist here is that you program it via the web. THe compiler resides in the "cloud." If you fill out a form on the contest page they will send you one of the boards absolutely FREE. I got mine last week. If someone likes your entry, you could win cash. If you write an article about it you could get a job offer, or even a phone call from a CEO.

NXP mbed Design Challenge

Yes, I'm going to enter something. It will have tubes and chips, but it won't be a stereo amp.

They ran out. so no more "free"
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