Rotel rb850

[gni]

Short the Feedback resistor = mute?

 

[AKN]

Thanks for mail, Chris!

Short the Feedback resistor = mute?

You are teasing now, right?

 

[gni]

No. . . .is seems that with 100% NFB the amp would just sit there
since the input to the LTP would be canceled by the output into
the other half of the LTP.

 

[AKN]

No

Yes

To put it short: Full feedback can only take away all gain, not whats given to the input. Result: Gain=1
Think of an op amp buffer.
And remember - stability issues with amp that is not unconditionally stable at all gain settings.

I have to sleep now, over midnight here in sweden. [

 

[gni]

goodnight - 3:16 PM Wednesday Afternoon. . .

http://www.wunderground.com/cgi-bin/findweather/getForecast?query=95492

 

[AKN]

Hi,

Chris, many thanks for the manual.

Late again 00:05, Thursday > Friday.

http://www.wunderground.com/global/stations/02446.html

 

[gni]

Cool weather for fall. . . we think 75 degrees F is cold. . . .we will
have a few more days of 90 degrees F. . .

Glad the Manual arrived. . .there is one error on the schematic. . .

C615/C616 should connect to circuit shared by:

VR601/VR602, Q611, Q617, etc.

 

[AKN]

there is one error on the schematic. . .

C615/C616 should connect to circuit shared by:

VR601/VR602, Q611, Q617, etc.

Yes that is correct, a missing dot above Q611 and 612.

But if one accepts the missing dot in the schematic...one should not even open the cover of an amp

Can you imagine, there has been an intense debate about this missing dot in an earlier thread.

 

[gni]

Fixed the dot on my copy. . . I know just enough to get myself
into trouble. . .

 

[gni]

New Bias Resistor and trimpot

Against anatech's advice (sorry) I did install the multi-turn bias pot.
For the 'reason' that the amount of DC current in the circuit through
R619+VR601 is very small. The measurement across these two
devices is the same points as the voltage of (be).

Left Channel:R619+VR601
Right Channel:R620+VR602

Left DC offset: +31.8mV
Right DC offset: +24.4mV

The old circuit path was R619 = 2.7kohms.
The new cirucit path is R619 = 3.58kohms.

The old VR601 was a 2.2kohm single turn.
The new VR601 is a 500 ohm multi-turn.

Spec is at 4mV across the 0.22ohm output resistor for 18.2mA.
Left
R619+VR601=3989ohms with 154.2microamps (615mV)
VR601=408.6ohms
Right
R620+VR602=3872ohms with 157.8microamps (611mV)
VR602=291.5ohms


Spec+8mV = 12mV across the 0.22ohm output resistor for 54.5mA
Left
R619+VR601=3752ohms with 162.6microamps (610mV)
VR601=172ohms
Right
R620+VR602=3670ohms with 165.9microamps (604mV)
VR602=90.4ohms

 

[anatech]

Hi Chris,
Please check my reply in the other thread.

Like not changing the oil in your car, this may not get you right away. It will probably get you at some point in time. I don't understand your insistence on using multi turn controls though.

This isn't the first time I've seen this issue come up. Many years ago I did get a rash of amps in that some idiot technician was installing as an upgrade. The controls became intermittent after some time. Those customers were not happy.

Please, do some investigating on these parts. They are simply not intended for your application. I have even had to replace them in test equipment due to age and intermittent operation. These were set and locked (so never moved). They are no better than 270 ° types, I feel they are worse.

-Chris

 

[gni]

I'm adding this post from the other post. . .since it belongs here.

This is a discussion on multi-turn pots mainly involving Anatach and Chris: in reference to the bias circuit in the RB850. Chris is responding
to Anatech's comments

Anatech responed to some measurements I posted.

Hi Chris,
It's the base current in the VBE multiplier you should be calculating, this is what passes through the moving contact.

Chris' Reply

I set the bias to 4mV on both channels.

The 2SD600+R618+R620+VR602 is the Vbe multiplier.
(with a constant current source).

I measured the voltage across R620 = 552mV
I measured the voltage across R620+VR602 = 615mV
The voltage potential across the base+emitter of 2SD600 is 615mV.

R620=3.58kohm = 154microamps

If 154microamps is flowing through R620, then it is flowing through
VR602 also.

VR602 voltage: 615mV-552mV = 63mV

Thus VR602 resistance is: 63mV / 154microamps = 408 ohms

Only 154microamps flows through the moving contact in spec mode!

Anatech

Your range of control is far too wide. If you allow the VBE multiplier to turn off, the amp goes boom. It does this in Technicolor and Cinemascope. Complete with stereophonic sound.

Chris' Reply

The original circuit had a 2.7kohm + 2.2kohm trim-pot = 4.9kohm
which was more than enough to shut down the bias. Which when
the trim-pot was set to 0 ohms was enough to run the bias up
to over 50mV.

The new circuit is a 3.58kohm + 500ohm trim-pot = 4.08kohm
This is a much smaller range with an upper limit almost 900 ohms
less than the original circuit. . . and a minimum resistance higher
than the original lowest resistance. . .the amp cannot go past
30mV bias now. It would be difficult to shut the bias down since
the upper resistance limit is 900 ohms less than the original design.
The control range of the new circuit is far less wide than the original.

Anatech

Your adjustment range will always keep the VBE multiplier in it's active range. There is no reason the circuit can not be designed with a restricted but useful range.

Chris' Reply

I agree. I think we are in agreement? All except the multi-turn pot.

I think it could be stated.

A multi-turn pot will require more maintenance in the future.
(due to DC current is detrimental to the small contact area)
A multi-turn pot will give higher precision adjustment of bias.

A single turn pot will give longer service life.
(due to a larger surface contact area)
A single turn pot will give lower precision adjustment of bias.
(exposed contact area can get dusty and imprecise)

Anatech and I will probably never agree on this subject. He has
seen more multi-turn pots in for service and seen the detrimental
effect of them in circuits with DC current.

I am willing to suffer the hair pulling of a multi-turn pot going
intermittent and replacing it; I want the precision--and with it,
suffer the ill effects.

I think I stated my position. This is a great discussion. . .

I hope this all makes sense. . .

 

[anatech]

Thanks for tying this all together Chris. I was running out of energy.

-Chris

 

[gni]

I've tied up your experience too long. . . there are plenty of other
DIY hackers out there that need your help too. . . should there be
a new thread specifically for the purpose of using the correct parts
in circuits. . . ?

Examples:

Proper use of Multi-turn pots in AC circuits.

Capacitor selection in audio circuits.

Grounding schemes for audio circuits.

Potentiometers as volume controls.

 

[anatech]

Hi Chris,
"Electronic and Parts" forum.

The manufacturer's app notes and other documentation are great sources of information. Just think. App notes are written by engineers who work with the parts and circuits on a full time basis. The education lying around is there for free! It only takes a little of your time. The only thing better is to play on your bench. But do heed recommendations given in app notes. I have not seen any that were greatly wrong, ever!

-Chris