Can this set up increase the output power of LM1875 at 4 ohms??

Leolabs · 17th November 2007, 01:01 AM

A single LM1875 will produce 20W into 8/4 ohms at +/-20V.Wonder if add a pair of current booster,will the output power at 4 ohms be increase to,let say 40W at 4 ohms at +/-20V???

Below is an example circuit:

consort_ee_um · 17th November 2007, 08:36 AM

This circuit is very close to the datasheet recommended booster circuit for the similar TDA2030. I have used the booster with both types of devices. You do not need R6 and R7 (can be replaced with short circuit)

richie00boy · 17th November 2007, 08:57 AM

The limiting factor is the supply voltage which is dictated by the chip. All you can do by adding booster transistors is run the chip at its max voltage, which it is probably only rated at 8 ohms for. So you will see a small gain in power but not huge.

Paswa · 17th November 2007, 09:42 AM

the main reason 4 using the chip, because it creates very low distortion. If u use the booster transistor, the output'll be more distorted than the single chip...

Anthony C Smith · 17th November 2007, 10:02 AM

this circuit is good for 35w RMS into 4r at low thd- cannot comment on the layout; when I have used this topology I always put the 1875 in the middle;looks better and lays out with shorter paths
I use 1R5 base resistors and a 220n cap across the 1875 rails and no 0.2r tail resistors; you will also need an input capacitor across the 47k Rin to prevent emi RFI suseptability (I use 22k not 47k and 470pF)
otherwise this is a good device use.
In a former life I used a version of this bridged to deliver 100W into 1R from a single 22V rail for voice alarm with no problems other than heat.

Leolabs · 17th November 2007, 02:24 PM

Can smaller power BJT such as TIP41/TIP42 be used???

Juergen Knoop · 17th November 2007, 05:55 PM

use at least the 'a' version. The basic TIP41 has a too low voltage rating.
My boosted TDA2030 utilises BD711/712 which are slightly more powerful than the TIPs (Pt 75W instead 65W).
Regards

Anthony C Smith · 25th November 2007, 11:29 AM

I have always used BD911 & BD912 as the ST ones are cheap and have 15A Ic Limit.
regards

17th November 2007, 08:57 AM	#3
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	The limiting factor is the supply voltage which is dictated by the chip. All you can do by adding booster transistors is run the chip at its max voltage, which it is probably only rated at 8 ohms for. So you will see a small gain in power but not huge. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

17th November 2007, 10:02 AM	#5
Anthony C Smith diyAudio Member Join Date: Jun 2005 Location: Sunderland	this circuit is good for 35w RMS into 4r at low thd- cannot comment on the layout; when I have used this topology I always put the 1875 in the middle;looks better and lays out with shorter paths I use 1R5 base resistors and a 220n cap across the 1875 rails and no 0.2r tail resistors; you will also need an input capacitor across the 47k Rin to prevent emi RFI suseptability (I use 22k not 47k and 470pF) otherwise this is a good device use. In a former life I used a version of this bridged to deliver 100W into 1R from a single 22V rail for voice alarm with no problems other than heat. __________________ Anthony C Smith MAES MinstSCE -I think rain is wet so who am I to Judge-

25th November 2007, 11:29 AM	#8
Anthony C Smith diyAudio Member Join Date: Jun 2005 Location: Sunderland	I have always used BD911 & BD912 as the ST ones are cheap and have 15A Ic Limit. regards __________________ Anthony C Smith MAES MinstSCE -I think rain is wet so who am I to Judge-

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17th November 2007, 08:36 AM	#2
consort_ee_um diyAudio Member Join Date: May 2004 Location: Hertfordshire	This circuit is very close to the datasheet recommended booster circuit for the similar TDA2030. I have used the booster with both types of devices. You do not need R6 and R7 (can be replaced with short circuit)

17th November 2007, 09:42 AM	#4
Paswa diyAudio Member Join Date: Aug 2007	the main reason 4 using the chip, because it creates very low distortion. If u use the booster transistor, the output'll be more distorted than the single chip...

17th November 2007, 02:24 PM	#6
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Can smaller power BJT such as TIP41/TIP42 be used???

17th November 2007, 05:55 PM	#7
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	use at least the 'a' version. The basic TIP41 has a too low voltage rating. My boosted TDA2030 utilises BD711/712 which are slightly more powerful than the TIPs (Pt 75W instead 65W). Regards

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