View Full Version : Preamps - Interesting exercise.

03-31-2013, 07:17 PM
Hi all. I have an old Phase Linear 400 and 2000 pre amp running into a pair of Yamaha NS1000's.
Front end is an ipod and Marantz SACD player.
My preamp has started to "plump" - that is it feeds DC into the power amp - not good.
So being the resourceful soul that I am not - found a solution. I connected the IPOD and CD player directly to the power amp, bypassing the preamp altogether. It works because the power amp has volume controls. IPOD's also have a volume and tone controls. Careful you don't short the input to the power amp - makes really nasty noises.
If you don't need tone controls you could replace the preamp with a switch with several inputs.
I cant hear a sonic difference but in theory less circuitry means better sound.
My 2 cents worth.

04-23-2013, 12:46 PM
Hi sweet212,

I had simular problem with a high level thump from the preamp section of a JBL SA660, when the amp was first turned on. The preamp section used a +23 volt supply that was generated from a +41.5V supply using a resistor and zener diode whose voltage was buffered by a NPN transistor. The emitter of the transistor is the +23V supply for the preamp.

There were two 1,000uF caps, one on the base and the other on the emitter to filter the +23V. The problem was the two caps were no longer 1,000uF. The cap on the base was very small with high leakage.

Replaced both caps and the thump went away. The cap on the base controlled how quickly the +23V would come up. With a good cap on the base the +23V came up below the low frequency cut off of the amp.

Check your power supplies. They should come up slowly.

04-23-2013, 03:49 PM
Slight thread hi jack , welcome to the forum Gordon. I see you live in Nevada City , one of my favourite places in Ca
. I'm guessing you are of an age since you know your way around an SA 660 , did you ever know Danny that ran Mad Dogs in Spring St ? I worked with him when he was house painting.
I have a cousin lives in Alta Sierra