Tube amp as signal processor (2x60W changes to 2x250W)
I have found that an optimum balance between tube sound and solid state sound can be achieved through the use of a good tube amplifier as a hifi signal processor (sound processor), also see the pictures posted below. The tube amp is then coupled to a so-called dynamic load instead of speakers, and a line output circuit is made from the speaker outputs L and R. This line signal is finally fed to the input of a solid state amplifier. The whole system is very easy to set up. One big advantage is the almost unlimited power and broad selection of suitable speakers.
While probably any tube amp can be used, I would especially recommend the fantastic chinese Music Angel KT88 amplifier (also known as model XD-800) – it sounds like a vintage Harman Kardon Citation II amplifier (and most chinese amps do not !). This amp is very popular since it is rather cheap costing US $ 400 from Hong Kong and seems to be very reliable. The internal construction is complex and containing lots of separate power supplies.
As a dynamic load the Weber Stereo Mass (a “speaker motor”) was tested and found to interact very favorably with the amplifier. When it was replaced by a 8 ohm power resistor the sound became thin and brittle – of no use.
The 4 ohm speaker outputs are taken to the Weber load, while the 8 ohms are used as signal outputs to next (solid state high power) amplifier - yielding line level through a resistor network (a voltage divider), see the figures at the bottom of this post.
The Weber Stereo Mass also has a set of speaker outputs (for reduced power); these are not used here. So the two big pots are instead used as an adjustment of the load in the treble region – they should be turned maximum counter-clockwise, then can be used for fine tuning of the treble region. Both switches should be set in the down position; the upper is a bypass switch (note that a tube amp should never run without a load !), the lower is not used in the present application.
Info on the various Weber units:
http://www.tedweber.com/atten.htm
You never get a transistor amp sound like this - extremely dynamic and full tube sound !
Pictures from left to right:
1) Weber Stereo Mass dynamic load
2) Internal circuit of the Weber unit
3) System set up with Music Angel tube amp as processor
Later added:
4) Shown here is a simple diagram of the actual voltage divider as used by this author. I have kept two of the long threads from the resistors and simply fixed them directly to the speaker output connectors. If a ground lift is needed (to remove hum loops), a 100 ohm resistor can be added to the amp negative output (earth, 0 ohm). I did not experience any problem. All resistors should be 1/2W Allen-Bradley or similar carbon composition. Avoid metal film and metal oxide in passive circuits.
I have found that an optimum balance between tube sound and solid state sound can be achieved through the use of a good tube amplifier as a hifi signal processor (sound processor), also see the pictures posted below. The tube amp is then coupled to a so-called dynamic load instead of speakers, and a line output circuit is made from the speaker outputs L and R. This line signal is finally fed to the input of a solid state amplifier. The whole system is very easy to set up. One big advantage is the almost unlimited power and broad selection of suitable speakers.
While probably any tube amp can be used, I would especially recommend the fantastic chinese Music Angel KT88 amplifier (also known as model XD-800) – it sounds like a vintage Harman Kardon Citation II amplifier (and most chinese amps do not !). This amp is very popular since it is rather cheap costing US $ 400 from Hong Kong and seems to be very reliable. The internal construction is complex and containing lots of separate power supplies.
As a dynamic load the Weber Stereo Mass (a “speaker motor”) was tested and found to interact very favorably with the amplifier. When it was replaced by a 8 ohm power resistor the sound became thin and brittle – of no use.
The 4 ohm speaker outputs are taken to the Weber load, while the 8 ohms are used as signal outputs to next (solid state high power) amplifier - yielding line level through a resistor network (a voltage divider), see the figures at the bottom of this post.
The Weber Stereo Mass also has a set of speaker outputs (for reduced power); these are not used here. So the two big pots are instead used as an adjustment of the load in the treble region – they should be turned maximum counter-clockwise, then can be used for fine tuning of the treble region. Both switches should be set in the down position; the upper is a bypass switch (note that a tube amp should never run without a load !), the lower is not used in the present application.
Info on the various Weber units:
http://www.tedweber.com/atten.htm
You never get a transistor amp sound like this - extremely dynamic and full tube sound !
Pictures from left to right:
1) Weber Stereo Mass dynamic load
2) Internal circuit of the Weber unit
3) System set up with Music Angel tube amp as processor
Later added:
4) Shown here is a simple diagram of the actual voltage divider as used by this author. I have kept two of the long threads from the resistors and simply fixed them directly to the speaker output connectors. If a ground lift is needed (to remove hum loops), a 100 ohm resistor can be added to the amp negative output (earth, 0 ohm). I did not experience any problem. All resistors should be 1/2W Allen-Bradley or similar carbon composition. Avoid metal film and metal oxide in passive circuits.
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