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The author builds and evaluates simplified common-source and current-feedback approaches, then modifies the design and ultimately returns to a strategy combining negative feedback with positive current feedback to drive output impedance toward zero for improved transient and frequency 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is the main motivation for developing the F7 after the F5?","Question",{"text":61,"@type":62},"The author aimed for better sound and an even simpler circuit than the F5, while returning to the F5 to see how it could be improved.","Answer",{"name":64,"@type":59,"acceptedAnswer":65},"How did the simplified circuit concept for the F7 work?",{"text":66,"@type":62},"The design targets a minimal amplifier with only four transistors and four resistors, using common-source operation for voltage and current gain and applying current feedback via the JFET source pins.",{"name":68,"@type":59,"acceptedAnswer":69},"Why is output impedance considered a key limitation in the earlier amplifier approach?",{"text":70,"@type":62},"The text explains that output impedance (inverse of damping factor) affects the flatness of response as load impedance varies and is important for transient response with reactive loudspeakers.",{"name":72,"@type":59,"acceptedAnswer":73},"What feedback balance does the F7 strategy use to improve performance?",{"text":74,"@type":62},"It balances modest negative feedback with a small amount of positive current feedback, creating an equilibrium that drives output impedance toward zero for improved transient and frequency response.","https://schema.org",{"og:url":32,"og:type":77,"og:title":10,"og:site_name":45,"og:description":12},"article",{"robots":79,"canonical":32},"index,follow",{"doc_id":81,"site_id":7},638596,1792689582,{"code":4,"msg":84,"data":85},"success",[86,90,94,98,103,107,112,117,122,125,129],{"id":22,"doc_module":4,"doc_module_name":25,"category_name":87,"show_sort_weight":88,"slug":89},"Story & 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\nIntroduction-Short Story Long:  \nConceived in 2007, the F5 was a push-pull Class A ampliﬁer employing eight semiconductors and 23 resistors to achieve 25 watts output with goodspeciﬁcations and good sound. You can read about it in detail in both the owner's manual and DIY construction article, both posted at www.ﬁ [rstwatt.com](rstwatt.com)  \nThe F5 received good reviews, sold well and received plenty of attention from the DIY audio community (as of this writing, the F5 thread [on](on www.diyaudio.com)[ ](on www.diyaudio.com)[www.diyaudio.com](on www.diyaudio.com)  [is cresting 3 million views](is cresting 3 million views)) . Like other First Watt ampliﬁers, it was a limited release, and was discontinued after 100 pieces were built.  \nSubsequently there was a more powerful version released to the DIYers, the F5 Turbo, which offered more power and numerous options for experimentation.  \nThe F5 was noted for detail and neutrality, the result of simple Class A operation, wide bandwidth, and a generous amount of negative feedback. With some material and equipment, the sound was exquisite , but the ampliﬁer also tended to highlight the faults of recordings and the rest of t he system. A poorly engineered record, a second-rate DAC or peaky loudspeakers and the ampliﬁer was merciless.  \nSince then I have spent time mostly exploring other kinds of design approaches to ampliﬁers, but I always meant to come back to the F5 to see how it could be improved. Mostly I was looking for two things – better sound and an even simpler circuit. The desire for a simpler circuit is self explanatory – apart from the aesthetic, I imagine that simpler circuits tend to sound better. (Because I build them myself, I will add cheapness and laziness to my motivations. )  \nI envisioned a circuit with only four transistors and four resistors – the bare minimum for an ampliﬁer of this type, where all the Fets are operated in Common-Source mode, giving both voltage and current gain. Two complementary input Jfets drive two complementary power Mosfets, and the output voltage is fed back to the Source pins of the Jfets in what is commonly called “Current Feedback”(CFA) .  \nThe schematic looked like this:  \nIn such a simple circuit , there are opportunities for improving performance by careful choice of transistors, resistor values, voltage and current values and precise matching of parts. I built up a number of such ampliﬁers evaluating both the measurements and the sound, and after a while a very nice little ampliﬁer emerged. That it (and the F5) resembled Plantefeve's Profet was testament to his ﬁne work.  \nIt had most of what was on my wish list: Very wide bandwidth Low distortion and noise  \nLarge Class A operating region  \nLess feedback  \nNo degeneration in the output stage  \nVery low thermal distortion and drift  \nNo capacitors or transformers (apart from the power supply)  \nLooks like a great laundry list. I thought the ampliﬁer sounded pretty good, but after a time the consensus was that it was kind of polite, not as musically involving as some other examples. One area where the ampliﬁer fell“objectively” short was the output impedance. Most of your “better” ampliﬁers have higher damping factors like 20 or 100, and this ampliﬁer was only about 5 or so, typical of a simple Common-Source topology and low negative feedback.  \nThe damping factor, which is the inverse of the output impedance , determines the ﬂatness of the ampliﬁer response when the load impedance varies and is important to the transient response of reactive loudspeakers, which is just about all of them.  \nYou could argue that single-ended tube ampliﬁers sound good and don't have much damping factor, and you would be right , but I'm not trying to duplicate single-ended tube ampliﬁers here – the First Watt SIT a mpliﬁers, doing a decent solid-state emulation of Triodes, represent that genre nicely. I'm shooting more for the sound of ","cbCaipXr7yI6pmYx","https://ap.wps.com/l/cbCaipXr7yI6pmYx","pdf",965017,12,"English","# Introduction - Short Story Long\n## Background on the F5 and F5 Turbo\n## Goals for improvement: sound and simpler circuit\n## Circuit design and evaluation\n## Output impedance, damping factor, and feedback strategy\n## Dance analogy: voltage source vs. F7 approach","[{\"question\":\"What is the main motivation for developing the F7 after the F5?\",\"answer\":\"The author aimed for better sound and an even simpler circuit than the F5, while returning to the F5 to see how it could be improved.\"},{\"question\":\"How did the simplified circuit concept for the F7 work?\",\"answer\":\"The design targets a minimal amplifier with only four transistors and four resistors, using common-source operation for voltage and current gain and applying current feedback via the JFET source pins.\"},{\"question\":\"Why is output impedance considered a key limitation in the earlier amplifier approach?\",\"answer\":\"The text explains that output impedance (inverse of damping factor) affects the flatness of response as load impedance varies and is important for transient response with reactive loudspeakers.\"},{\"question\":\"What feedback balance does the F7 strategy use to improve performance?\",\"answer\":\"It balances modest negative feedback with a small amount of positive current feedback, creating an equilibrium that drives output impedance toward zero for improved transient and frequency response.\"}]","Now For Something Completely Different - the F7 Power Amplifier | PDF"]