DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 17 Feb 2018, 16:25

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 4 posts ] 
Author Message
PostPosted: 24 Feb 2017, 16:29 
Offline

Joined: 24 Feb 2017, 16:20
Posts: 2
Hi,

I have recently purchased an amplifier module based on the TDA8950 device from eBay and am experiencing distortion at all output levels.
(eBay: http://www.ebay.co.uk/itm/252534422644)

The module includes a bridge rectifier and smoothing caps and only requires a (stated) 22V-0-22V AC supply.

The circuit board does not appear to have any form of voltage regulation and according to the NXP spec sheet the device can handle a wide range of supply voltages from ±12.5 (min) ±35 (typ) ±40 (max).
(NXP spec sheet here: http://www.nxp.com/documents/data_sheet/TDA8950.pdf

I have a been powering it with a toroidal transformer supplying ±30V AC. (Iv'e also swapped the transformer for a 21-0-21V one but the distortion is still present.)
When I first set it up on the bench I connected it to a small set of 4.8Ω speakers and fed it with a line level signal from a CD player. At all volume levels there is distortion.

If I set up the amplifier with a signal generator producing a 1KHz test tone at 0.4V and drive 4.7Ω power resistors (or the speakers) and look at the input and output signals on an oscilloscope the output trace apears ‘fat’ when compared to the input. The output is not clipping - although that can be achieved.

The user connections on the board are:
AC Power In: 22V-0-22V AC
Signal Input: (via what looks like a JST type connector) Left-Ground-Right
Outputs: Out-L+GND & Out-R+GND

There was no supporting paperwork supplied with the module. (I have briefly described the problem to the Chinese supplier and their brief response was “friend, could you check your connect and try it again”)

Any ideas and thoughts would be most welcome.

Thanks


Top
 Profile  
 
PostPosted: 27 Feb 2017, 12:26 
Offline
User avatar

Joined: 05 Aug 2016, 14:35
Posts: 215
first, and for the record, I do not like the Molex Connector input. That is a lot of unshielded metal in a very sensitive spot.

In no particular order, I suggest the following:

a) Shield the inputs.
b) Make sure that input voltage is no more than 1V for now. By actual test. If you have access to a variable-output pre-amp, use that, and very gradually increase input until you observe distortion.
c) Try a high-impedance source if available, with a variable output, as above.
d) Make sure the board is shielded, away from magnets, and cool.
e) Make sure your power-supply is capable of driving this device. A potential of 240 watts from a net-52V input means "about" 5A necessary from the power transformer.

I am working around the most common problem such as you describe as - short of bad parts - being overloading on the input side.

After which, without it on the bench in front of me. I have no other suggestions.


Top
 Profile  
 
PostPosted: 27 Feb 2017, 14:53 
Offline

Joined: 24 Feb 2017, 16:20
Posts: 2
Hi Peter, Many thanks for this..

a) The input cable is shielded up until about 8cm of wire that are solder tacked onto the solder side of the input (Molex) as there was no input cable supplied. I can solder shielded directly to the PCB if required. Is that what you mean?

b) My workshop sound In/Out is an elderly Soundcraft Delta which of course has variable balanced outputs. In the this case and with other non balanced i/o I simply connect pins 1 GND and 2 +Signal to the input of the device under test. Is this OK?
b.2) In the case of the function gen, this is connected directly to the amp i/p

c) The spec of the Delta states that the output impedance is 75Ω. As stated here: https://dl.dropboxusercontent.com/u/145 ... 281%29.pdf

d) Do you mean I should encase the module in a shielded box for the tests?

e) I have a 50VA and a 35VA transformers to hand at the moment to power the module and in it's working life it will be running at low o/p levels. Could these be the issue?

Thanks again for your help.


Top
 Profile  
 
PostPosted: 27 Feb 2017, 15:44 
Offline
User avatar

Joined: 05 Aug 2016, 14:35
Posts: 215
Please note the interpolations.

DanSteely wrote:
Hi Peter, Many thanks for this..

a) The input cable is shielded up until about 8cm of wire that are solder tacked onto the solder side of the input (Molex) as there was no input cable supplied. I can solder shielded directly to the PCB if required. Is that what you mean?

8 cm is a bit over 3" in the old currency. WAY too long to be unshielded. Going directly to the PCB is desirable, however.

b) My workshop sound In/Out is an elderly Soundcraft Delta which of course has variable balanced outputs. In the this case and with other non balanced i/o I simply connect pins 1 GND and 2 +Signal to the input of the device under test. Is this OK?

Do you have an audio source that you could use temporarily? Something at/around a 1.5 - 2V output at/above a 1,000Ω output impedance?

b.2) In the case of the function gen, this is connected directly to the amp i/p

c) The spec of the Delta states that the output impedance is 75Ω. As stated here: https://dl.dropboxusercontent.com/u/145 ... 281%29.pdf

Probably way too low for this device. Again, I am only guessing, but an audio amp wants a higher impedance input, normally.

d) Do you mean I should encase the module in a shielded box for the tests?

Can't hurt, might help. But given the 8cm of unshielded input cable, shielding the board gets less important at this moment.

e) I have a 50VA and a 35VA transformers to hand at the moment to power the module and in it's working life it will be running at low o/p levels. Could these be the issue?

Both devices are _WAY_ too small. Even at a power-factor of 1 (no losses), you are getting at best 50 watts into a device that wants more. Voltage sagging will cause distortion. Do either of the transformers heat up when in use? No, you cannot (certainly should not) parallel the transformers.

Thanks again for your help.


Between the efficiency losses in Class D, and the requirements of the amplifier, I expect that you are getting voltage sag, and that is the probable cause of the distortion. I cannot be certain, again without the device in front of me. But, you could put a VOM on the transformer and determine the actual voltages loaded and unloaded. That would be revealing.

The bottom line is that you will need 24V @ 5A of output per side, or 10A in total to drive that board. 24@ 5A - 0 - 24@ 5A


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 11 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
cron
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy