Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jul Tue 23, 2019 2:00 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 12 posts ] 
Author Message
 Post subject: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sat 01, 2019 5:55 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2766
Location: Burbank, CA and Thailand
Mods: I'm not sure what forum this belongs in, sorry if this was the wrong choice.

While doing a minor mod on my beloved B&K AVR202 receiver I accidentally damaged a PCB and am in the middle of repairing it. I think I'm almost done, but am waiting for a 74HCU04 that I had to order.

I had never encountered that logic family before and tried using a plain old 74HC04 at first. It didn't work.

I found a little info online about this logic family but don't really understand how or why it is used. In the circuit I'm servicing, the gates in this IC are used in a circuit that selects among the various coax and optical digital 5.1 inputs. At least one of these inverter gates has a feedback resistor between input and output.

As I understand it, the unbuffered outputs allow passage of analog signals. But the signals in this case aren't really analog -- they are digital audio, though they are "raw", having come in from the input jacks without yet being conditioned by any other logic gates. I'm having trouble wrapping my head around the reasons for the unbuffered gates in this application, and don't understand the reasons for the feedback resistors.

If anyone could help with some kind of tutorial, I'd appreciate it. I've worked with logic gates for decades but this is new to me. I can post a schematic if it helps.

Thanks in advance.

_________________
My High Voltage Capacitor Store
ARF members: PM me for 20% off
Click Here


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sat 01, 2019 7:54 pm 
Member

Joined: May Wed 23, 2018 6:28 am
Posts: 402
This should help you:

http://melbmcu.weebly.com/cmos-gate-as-analogue-amplifier.html

The 74HC04 is a pure digital part. The 74HCU can be tricked into variable trigger and output points, creating an amplifier or oscillator. Feedback is a characteristic of, these. An oscillator is like an amplifier with too much feedback. The buffering in the 74HC04 prevents using this trick. I don't know if this article mentions it, but you can use multiple stages to clean up the noise, which may be good or bad, depending on the application. As an amplifier, it has a lower frequency limit.

Attachment:
74hcu.jpg
74hcu.jpg [ 311.33 KiB | Viewed 487 times ]


John


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sat 01, 2019 10:23 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2766
Location: Burbank, CA and Thailand
Thanks, that does clarify this for me, though I'm still not sure why the designer chose this particular device for the circuit. Do you think it was to level shift and/or clean up the digital signals coming into the receiver from external sources, like in the screen shots you posted? Here's the schematic. (BTW, U1 is an analog switch.)

Image

_________________
My High Voltage Capacitor Store
ARF members: PM me for 20% off
Click Here


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sun 02, 2019 1:17 am 
Member

Joined: May Wed 23, 2018 6:28 am
Posts: 402
I'm as surprised as you to find it in a high end receiver, but then my real hardware design years ended 20 years ago, so I haven't kept up with it. If someone wanted me to design a preamp with an S/N of 89dB, the last place I would have thought to go is a digital chip turned into an analog amp. I might have thought to use it in a lesser amp. Putting my ignorance aside, it conveniently has the 6 amps needed for the job, does the level shifting and cleans up the noise in the second stage. Its frequency limit certainly isn't an issue either.

Thanks for posting the schematic. It may be useful to me.

Good luck
John


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sun 02, 2019 1:22 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2766
Location: Burbank, CA and Thailand
In case I was unclear before, the 74HCU04 gates apparently are being used as analog amplifiers here, but signals passing through them are 5.1 serial digital audio (from optical or coax), not analog audio.

_________________
My High Voltage Capacitor Store
ARF members: PM me for 20% off
Click Here


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sun 02, 2019 1:52 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34374
Location: Maryland 20709, USA
I designed a LOT of equipment using 74xxx logic back in the day (witness my 13-page resume).
I don't recall ever seeing the 74HCU family. There may have been only this one device.

It appears to be just a regular op amp modified to work from a unipolar 5-volt power source rather
that the normal +/-15 volt supplies.

- Leigh

_________________
73 de W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sun 02, 2019 2:37 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5249
Location: Rochester NY USA
The buffered CMOS inverters use three inverter stages per gate, the unbuffered only one. The unbuffered ones were the only types when the CMOS 4000 series was introduced and the buffered 4000B series replaced most. The buffered gates were faster with much better rise and fall times. In the meantime, several applications used CMOS as linear devices and oscillators but they would not work with 4000B series, so the 4000A series was continued for a few inverters and gates. 74HC series took over from 4000 series since it had better TTL compatibility and TTL pinouts. I guess it makes sense to have a unbuffered 74HCU04 for those few "linear" applications. A buffered gate will oscillate if a feedback resistor is used, near its maximum switching frequency.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sun 02, 2019 4:52 am 
Member

Joined: May Wed 23, 2018 6:28 am
Posts: 402
Normally, the feedback implies an amplifier or oscillator. I didn't look at the diagram closely enough earlier, so I looked again. It is an amplifier and filter, but you are correct that it isn't being used as a preamp. The first stage amplifies the SPDIF coax level to TTL level and the second stage cleans it up. The switch selects which coax. The simpler circuit below it is to raise the 0.5V TOSLINK SPDIF to TTL.

John


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sun 02, 2019 5:23 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2766
Location: Burbank, CA and Thailand
I checked but couldn't find any other 74HCUxx devices so Leigh, maybe you're right...this is the only one. Weird. I might play around with one on a breadboard to get a better idea about how it operates.

I think I understand the circuit pretty well now and am in a good position to make sure it's working right once my replacement 74HCU04 shows up. Thanks for the replies, which were very helpful.

_________________
My High Voltage Capacitor Store
ARF members: PM me for 20% off
Click Here


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Sun 02, 2019 5:59 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34374
Location: Maryland 20709, USA
Hi David,

The ON Semi datasheet has some sample circuitry.
https://datasheet4u.com/datasheet-pdf-f ... /74HCU04/1

This datasheet from NXP Semi has some additional circuits:
https://datasheet4u.com/datasheet-pdf-f ... /74HCU04/1

I think the NXP sheet has the most info useful for experimenting.
Both datasheets show the actual schematic of one inverter.
It's so simple (only two FETs) you might miss it.

- Leigh

_________________
73 de W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Mon 03, 2019 3:09 pm 
Member

Joined: Oct Tue 18, 2016 8:17 pm
Posts: 135
ARF's favorite youtuber, Mr Carlson, has an oscillator video that discusses this chip. About the 10 minute mark.

https://www.youtube.com/watch?v=tb1-EF_F2Rk


Top
 Profile  
 
 Post subject: Re: 74HCUxx "Unbuffered" Logic Family - Trying to Understand
PostPosted: Jun Thu 06, 2019 2:28 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2766
Location: Burbank, CA and Thailand
Leigh, thanks for finding the datasheet (I had one, but not as detailed) and jbrill, thanks for the video link. Definitely helpful.

I got my U04's today, popped one in, unit works perfectly now. Overall it was a fairly big job...power supply problem, burned foils, etc. The U04 was the last piece of the puzzle but the most puzzling, at least at first, before I understood how this odd little part was supposed to work.

It's funny how frustrating tech jobs can sometimes turn into cool little learning experiences.

Thanks again everyone for your help. It back where it belongs now.

Last year I fixed the clock in the photo. That was a pain and a learning experience too.

The B&K is part of a pretty elaborate system I've been building up over the last few years. It's set up for stereo, 5.1, and (wait for it!) quad, with 9 speakers that are selected in various combinations. Earlier this year I did a write up with photos on an audio forum where I hang out. Here's the link if anyone's interested: https://forums.stevehoffman.tv/threads/ ... t-20704724 My post is near the bottom of the page.

Image

_________________
My High Voltage Capacitor Store
ARF members: PM me for 20% off
Click Here


Top
 Profile  
 
Post New Topic Post Reply  [ 12 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 7 guests



Search for:
Jump to:  


























Privacy Policy :: Powered by phpBB