Go Back   Home > Forums > Design & Build > Construction Tips

Construction Tips Construction techniques and tips

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd July 2013, 07:37 AM   #1
diyAudio Member
 
Join Date: Mar 2011
Default Splitting balanced line level for passive bi-amping

My preamp has one set of single ended, and one set of balanced outputs to connect to a power amp. I've recently bought a 5 channel power amp and I would like to use 4 channels to try out passive bi-amping.

My speakers have proper tri-wire inputs where each is separate from the other. I would probably wire the bass to one channel and the mid and treble to another. I have discussed converting my speakers to active with my manufacturer (PMC) who also make actively powered studio monitors. They've said I'd need an extremely good active crossover and a lot of testing and equipment to get it to sound as good as their built in passive. They also pointed out the drivers were not selected or intended to be actively powered i.e. I think their inductances may be different.

So I know the standard way to split each output from the preamp is to use a 'Y' cable that connects the single ended output in parallel to 2 power amp inputs. However this effectively doubles the load on the preamp by halving the inductance at its output (correct me if I'm wrong).

Another way I thought I could do it would be to split the balanced output. My idea would be to connect one power amp channel to the hot and ground pin, and the other between the cold and ground. Chassis ground would be left unconnected. As each signal is out-of-phase, I would wire one of the outputs of the amplifier to the speaker out-of-phase to correct this. Obviously the signal wouldn't be balanced but it does avoid having to wire anything in parallel.

I can't find anything at all about anyone trying this on the web. Does anyone know if it might be a good idea or am I just showing my lack of knowledge on the subject?!
  Reply With Quote
Old 2nd July 2013, 08:23 AM   #2
diyAudio Member
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
The manufacturers would say that their system is going to work better than anyone elses'.
Using a 'Y' cable is probably good enough for what you want to acheive as the output impedance of your pre-amp should be a few KOhms and the input impedance of your power amps should be more than 18KOhms.
probably because they have spent quite a bit in R&D to get it right.
A balanced line out, is as it states, balanced but also floating with respect to ground so will not work as you suggest.
__________________
www.flyingmole.co.uk For World Wide support for Flying Mole, Class D, Refurbishment and Repairs. www.jonsnell.co.uk

Last edited by JonSnell Electronic; 2nd July 2013 at 08:24 AM. Reason: spurious letters
  Reply With Quote
Old 2nd July 2013, 11:44 AM   #3
diyAudio Member
 
Join Date: Mar 2011
Thanks for the tip. I had forgotten about balanced not being referenced to ground to be honest!

The output impedance of the pre is 47 ohms, and the input impedance of the power is 10K. Is that too low for bi-amping?
  Reply With Quote
Old 2nd July 2013, 12:45 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Two channels of paralleled 10k inputs will present 5K to your source.

The paralleled capacitances of the two cables (and the RF attenuation at the inputs of your amps) will present doubled capacitance and that will draw doubled current charging the capacitance.
The current to drive the cable will probably be more than that required to drive the 5k

Your 47ohm source will easily drive the 5k. 3Vpk into 5kohms is just over 0.5mApk
Check the source current specification to see if it can drive the cable properly.
__________________
regards Andrew T.

Last edited by AndrewT; 2nd July 2013 at 12:57 PM.
  Reply With Quote
Old 2nd July 2013, 04:58 PM   #5
diyAudio Member
 
Join Date: Mar 2011
The preamp manufacturer doesn't specify the current specification of the outputs.

However I was able to find this on the datasheet of chip used at the output stage:

Quote:
Analog Outputs

The analog outputs are capable of driving 2 kΩ loads to within 1.35 V of the analog supply rails and are short-circuit protected to 20 mA.

The minimum output load resistance is 2 kΩ; a load smaller than 2 kΩ may cause increased distortion. As the load resistance decreases, the potential for increased internal heating and the possibility of damage to the device is introduced. Additionally, the load capacitance should be less than 100 pF. Increased load capacitance may cause increased distortion, and the potential for instability in the output amplifiers.

If a low-impedance or high-capacitance load must be driven, an external amplifier should be used to isolate the outputs.


I'm not sure if an additional output stage is used. There is certainly at least a passive component in the signal since the output resistance of the chip is 100 ohms, but the preamp manufacturer states the output as 47 ohms.

The total capacitance of the 2 cables per used per output will be less than 64 pF.
  Reply With Quote
Old 3rd July 2013, 08:14 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
64pF is tiny, don't concern yourself with this.
The RF attenuation in the amp will probably be bigger.
__________________
regards Andrew T.
  Reply With Quote
Old 3rd July 2013, 04:03 PM   #7
diyAudio Member
 
Join Date: Mar 2011
Thanks. I just need to source some decent bi-wire speaker cables and then I'll solder up a pair of Y RCA/Phono cables. Thanks again to all for the help
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help with DIY passive balanced line level attenuator pads. massappeal129 Analog Line Level 1 20th August 2012 12:27 AM
Basic questions on passive line-level XOs and with bi-amping NebuK Analog Line Level 16 15th December 2011 02:27 PM
Passive bi-amping with gainclone?? steveline Chip Amps 21 22nd August 2008 09:09 PM
bi-amping through passive crossovers ultrachrome Multi-Way 4 31st March 2005 01:14 AM
passive bi-amping parts eduard Tubes / Valves 25 24th July 2003 11:19 PM


New To Site? Need Help?

All times are GMT. The time now is 09:57 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2