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Old 15th May 2003, 07:37 AM   #1
Flavius is offline Flavius  Romania
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Default sort of preamp for the leach?

Hello everybody !

I just got bitten and I must build one of those nice Leach Amps ! I will be either a 2 or 3 chanell depending on how much cash i have to build a sub too.
Means I will sell my current amp which is a Marantz PM 4200, and perhaps my speakers too. They way i made them, they have too much bass to really need a sub, but not exactly enough to have a good response below the 38Hz Fb

Besides these and the Marantz, I use a Marantz tuner, a Denon cd player and the computer (mp3/ DVD) as sources.

Now, If i toss the amp and build a Leach, I will probably need a preamp to handle volume and source switching, correct?

Prof. Leach explains a simple volume mod for the amp.
Lets assume I take a second chassis, add to it a bunch of inputs, a selector to switch between them, then the volume bit, and all this to the Leach inputs.
Would that work for a preamp ? I dont need tone correction. Perhaps put 2 pots left and right to adjust the balance too, and make separate inputs for the Leach.

Thanks for any ideeas.
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Old 16th May 2003, 03:14 AM   #2
Diode is offline Diode  United States
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Hey Flavius,

If I were you, I'd have at least unity gain buffers between the components and the amp. Better yet, I'd have variable gain op-amps to increase the input signal if need be.

Good luck getting switches. It sounds like a pain in the butt to me.

Good luck!

Chris
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Old 16th May 2003, 06:00 AM   #3
bawang is offline bawang  Malaysia
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Default Choice of preamps

You can choose any one of the many preamps mentioned in this forum, ie BOZ, DOZ, tubed types or even one of your own design. However, if you don't need additional gain or buffering, the easiest way is to make a passive preamp (as you've described, ie just source selection and passive volume control). If you choose the passive route, make sure that the source can handle the loading of the amp. For the Leach, your source must be able to put out around 2V maximum so that the amp can be driven to full power.
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Old 16th May 2003, 08:57 AM   #4
Flavius is offline Flavius  Romania
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Thank you guys. I checked the cdplayer specs, it says 0.2 - 2 V output (it has a volume control)
I know the SB Live! gives a roughly equal output, while the tuner is definatelly louder than the CD. I'll need to find the manual and have a look.

I suppose a gain circuit is supposed to boost the source signal? I dont really that, I guess. I dont have a clue about what a buffer does though.
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Old 18th May 2003, 01:53 AM   #5
bawang is offline bawang  Malaysia
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Hi again. The function of an active (ie powered) preamp is to act as gain and in most cases, a buffer for the sources. However, if your source can drive the input impedance of the main amp (and comes with a volume control), then it's OK to drive the amp directly. You can check the User's Manual of the source equipment, usually the output of the source will be spec'd as xx mV into yy kilo-ohms.

A "buffer" is to serve as an isolator between the source and amp, while allowing the signal to pass through unchanged (hopefully!) without loading the output stage of the source equipment.

Hope this helps.
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Old 26th May 2003, 08:01 AM   #6
Flavius is offline Flavius  Romania
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Thank you. I had a look at the doz preamp. The designer says the cdp bypasses the gain circuit and goes directly to the pot. That left me because with my sources, it is the tuner that is much louder than the cdp and the PC soundcard.
Assuming I start all 3 with a typicall content, put a decent ammount of volume from the amp, then start switching, the cdp and soundcard sound ok and roughly equal, but switching to radio - it will blow my head off !
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Old 28th May 2003, 08:30 AM   #7
bawang is offline bawang  Malaysia
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Unfortunately, this is one of the facts of life. You can always put a potentiometer at the output of your tuner to control the input to the preamp, that's what I'll do, provided I have the time, of course. For the time being, I'll just tolerate the volume differences..........
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Old 28th May 2003, 08:41 AM   #8
Flavius is offline Flavius  Romania
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Well I am used to it by now. Having the amp on my desk (and a remote anyway) I can reduce the amp volume before switching to radio. Im just curios if a Marantz cdp will sound on par with the tuner.


Regarding the actual construction, I was thinking at a single chassis to host both the preamp and the Leach, to cut down on costs and simplify construction. It will still have separate PCBs and PS's
I assume the use of separate chassis is to eliminate any interference between the preamp and amp, but then again a 1 cm hardwire between the 2 is more appealing than some long internal wires, RCA's and 50cm of interconnects.
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Old 28th May 2003, 09:43 PM   #9
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Leach amplifier need quality preamp!!! - that is fact

My CD player put out maximum 2.3V so I connected them with pot directly to Leach and sound was very thin, lack of energy and bass,also it was not easy to listen to.

Then I tried with valve preamp with 6 dB gain things get much better. Then I borrowed one solid state preamp with 15! dB gain and Leach sound was magnificient.

I would go for at least 10dB gain and forget passive preamp!!
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Old 30th May 2003, 04:36 AM   #10
bawang is offline bawang  Malaysia
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Flavius, if your casing is big enough (the Leach needs quite a large casing) and you take proper precautions about magnetic shielding and proper grounding, I don't see any problems about having the BOZ and Leach in tha same chassis. But I do expect problems, especially humming. Preamps are extremely sensitive to humming, especially when located near a large transformer (needed for a power amp), although attention to magnetic shielding can help. Personally, I will build the BOZ and Leach in separate casings.
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