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Old 9th February 2003, 02:31 PM   #1
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Question BOSOZ Volume Control - any suggestion

Any suggestion...?

I've put a 100K Volume control (ALPS) at the output of my BOSOZ, with any sources at all, I found the output level is too high i.e. only 1-2 clicks (less than 7:00AM)...much lounder than any Pre that I've been using (SFL-1 and DACT CT101).

R15 used in the circuit is 124Ohm, selector switch is used (at 0 and 100Ohm) instead of P5.

Anyone has any idea, which is better between input and output volume control...(I prefer to use one)? and Why?

Also, has anyone every tried to use RCA-XLR converter as the input, what is the outcome?

Thank You,
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Old 9th February 2003, 06:11 PM   #2
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Default Best Anttenuator

I found a vast improvement in sound quality when I configured a 23 postion four deck Electro switch so that at any position the source saw 10K0 via two resistors in series and the preamp saw just one resistor of the two resistors that the source saw (simple voltage divider).
To hear if there will be any improvement in sound in your system try dividing the souce voltage with two resistors and listening for an hour or so. Make sure you remove or bypass your original atteneuator first.
The improvement in sound quality should be dramatic. I think I still have the resistor values if you want them.

My favourite antenuator; however, only works for balanced preamps.
I use a 23 position switch which places only one resistor between the + and - leads of one channel. I do not think it is possible to get a more simplistic attenuator than this.
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Old 9th February 2003, 06:20 PM   #3
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Default Attenuator at output

I am sorry I forgot to answer your question in my previous post.

I prefer the attenuator at the output for the following reasons:

From my previous post you will see I use only one resistor between the + and - of one balanced channel.

If I put the attenuator on the input I have to protect the source by placing two 10K0 resistors in front of the attenuator.

This protects the source from being shorted out at some low volume settings.
This works great with balanced sources, but can sometimes lead to unexpected results with unbalanced sources.

Placing the attenuator on the output works well for both balanced and unbalanced sources and it also helps attenuate noise.
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Old 9th February 2003, 11:01 PM   #4
Pan is offline Pan  Sweden
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Default Ignatz

A maybe better yet variant of what you describe is this;

Balanced ouptut, one series resistor on each phase and one soldered resistor to GND on each phase. A stepped att. with one single extra resistor between the phases (as you describe).

The fixed paralelled resistors are matched for the max volume you need on a specific set-up.

The +´s with this solution is;
The signal partly goes thru a soldered path to GND and the effect of the contacts in the ´switch is minimized.
When listening at max volume all you have in the circuit is a soldered voltage divider.

Also the input capacitance of the amp sees a lower resistance to GND via the two paralelled resistors to GND instead of the singel resistor between the phases.

You can use three super-duper resistors for the fixed values and cheaper (if you want :-) ones in the switched attenuator.
An even cheaper solution is three quality metal films and a single pot of your choice.

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Old 9th February 2003, 11:29 PM   #5
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Default No resistor to ground

I am not sure exactly what you are saying, but I think this may be what you are talking about.

On the balanced attenuator there is no value in putting a resistor to ground. The resistor between + - simple adds more common mode which is then rejected by the balanced configuration of the amp.
Think of two sine waves 180 degrees out of phase. If you connected both outputs with a wire you would see zero voltage on your scope or metre because both sine waves are exactly 180 degrees out of phase at all times. The sum of the two amplitudes always adds to zero volts.

If you change the wire to a resistor you will see some signal but it will be attenuated porportional to the the inverse of the resistor value.

This is why this type of attenuator is so good with balanced signals.
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Old 9th February 2003, 11:37 PM   #6
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what about the simple ADC0804 based relais attenuator of the aleph p 1.0 ?
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Old 10th February 2003, 04:44 AM   #7
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Thank you guys...

I could find only this ALPS model (4-deck) from a local store. A bit difficult to find such stepped-attenoator, I do have DACT in my mind...I will try but may not be able to get one until March.
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Old 10th February 2003, 05:52 AM   #8
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Default Re: BOSOZ Volume Control - any suggestion

Originally posted by ruangrit
Any suggestion...?
1. Greater resistor bridged between sources, or
2. Addinitonal input volume

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Old 10th February 2003, 07:24 AM   #9
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Thanks but I'll try to avoid input attenuator to minimize wiring complexity (I planned to use balanced input as well).

In the bosoz construction manual there are statements of "not recommend exceeding 25KOhms for P3 & 4" and "lower values of output potentiometers will reduce the gain of the circuit"...

Be bare with my rusty electronics knowledge, taken a look at the circuit where output potentiometers are paralleled to R11 & 12 influencing output resistance, I do understand that the output impedance goes up when higher values is used but do not understand why the circuit gain is reduced.

Am I missing anything??
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Old 10th February 2003, 08:21 AM   #10
Fox is offline Fox  Netherlands
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Default Lower the gain

Hi RR,

When you have to much gain in the BOSOZ several options come into mind:

1. Take a higher value for R15. When the resistance approaches zero, the gain is approximately 30dB. You should take a value somewhere between 400 ohms and 500 ohms to set the gain to +/- 10dB of gain.

2. Place a 10 kohm pot at the input of the cicuit to lower the output voltage of your sources.

All this and more was also stated by Nelson Pass in the original BOSOZ article Good luck and post your future findings. I'm most interested because I'm will build a BOSOZ myself in the future.


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